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1911 Encyclopedia Britannica
Of the three names generally used for this order of reptiles, viz. tortoise, turtle and terrapin, the first is derived from the Old French word tortis, i.e. twisted, and was probably applied first to the common European species on account of its curiously bent forelegs. Turtle is believed to be a corruption of the same word, but the origin of the name terrapin is unknown: since the time of the navigators of the 16th century it has been in general use for fresh-water species of the tropics, and especially for those Of the New World. The name tortoise is now generally applied to the terrestrial members of this group of animals, and that of turtle to those which live in 'the sea or pass a great part of their existence in fresh water. They constitute one of the orders of reptiles, the Chelonia: toothless reptiles, with well developed limbs, with a dorsal and a ventral shell composed of numerous bony plates, large firmly fixed quadrates, a longitudinal anal opening and an unpaired copulatory organ.
The whole shell consists of the dorsal, more or less convex carapace and the ventral plastron, both portions being joined laterally by the so-called bridge. The carapace is (with the exception of Sphargis) formed by dermal ossifications which are arranged in regular series, viz. a median row (1 nuchal, mostly 8 neurals and 1-3 supracaudal or pygal plates), a right and left row of costal plates which surround and partly replace the ribs, and a considerable number (about 11 pairs) of marginal plates. The plastron consists of usually 9, rarely 11, dermal bones, viz. paired epi-, hyo-, hypoand xiphi-plastral plates and the unpaired endo-plastral; the latter is homologous with the interclavicle, the epi-plastra with the clavicles, the rest with so-called abdominal ribs of other reptiles.
In most Chelonians the bony shell is covered with a hard epidermal coat, which is divided into large shields, commonly called "tortoiseshell." These horny shields or scutes do not correspond in numbers and extent with the underlying bones, although there is a general, vague resemblance in their arrangement; for instance, there is a neural, a paired costal and a paired marginal series. The terminology may be learned from the accompanying illustrations (figs. i and 2). The integuments of the head, neck, tail and limbs are either soft and smooth or scaly or tubercular, frequently with small osseous nuclei.
All the bones of the skull are suturally united. The dentary portion of the mandible consists of one piece onl y, both halves being completely fused together. The pectoral arch remains separate in the median line; it consists of the coracoids, which slope backwards, and the scapulae, which stand upright and often abut against the inside of the first pair of costal plates. Near the glenoid cavity for the humerus arises from the scapula a long process which is directed transversely towards its fellow; it represents the acromial process of other vertebrates, although so much enlarged, and is neither the precoracoid, nor the clavicle, as stated by the thoughtless. The;tail is still best developed in the Chelydridae, shortest in the Trionychoidea. Since it contains the large copulatory organ, it is less reduced in the males. No Chelonians possess the slightest traces of teeth, but their jaws are provided with horny sheaths, with hard and sharp edges, forming a beak.
The number of Chelonians known at present may be estimated at about 200, the fresh-water species being far the most numerous, and are abundant in well-watered districts of the tropical and sub-tropical zones. Their number and variety decrease beyond the tropics, and in the north they disappear entirely about the both parallel in the western and about the 56th in the eastern hemisphere, whilst in the southern hemisphere the terrestrial forms seem to advance to 36Â° S. only. The marine turtles, which are spread over the whole of the equatorial and sub-tropical seas, sometimes stray beyond those limits. As in other orders Testudo pardalis, to show the divisions of are marked by entire lines, and of the being marked by dotted lines. Fig. i, Fig. 2, Lower or ventral aspect.
Bones of the Carapace :- co l, Costals.
m 1 , Marginals.
of reptiles, the most specialized and the largest forms are restricted to the tropics (with the exception of Macroclemmys); but, unlike lizards or snakes, Chelonians are unable to exist in sterile districts or at great altitudes.
They show a great divergence in their mode of life - some living constantly on land, others having partly terrestrial partly aquatic habits, others again rarely leaving the water or the sea. The first-mentioned, the land tortoises proper, have short club-shaped feet with blunt claws, and a very convex, heavy, completely ossified shell. In the fresh-water forms the joints of the limb bones are much more mobile, the digits distinct, armed with sharp claws, and united by a membrane or web; their shell is less convex, and is flattened, and more 'or less extensive areas may remain unossified, or transparent. windows are formed with age, for instance in Batagur. As a rule, the degree of development of the interdigital web and of convexity of the shell indicates the prevalence of aquatic or terrestrial habits of a species of terrapin. Finally, the marine turtles have paddle-shaped limbs resembling those of Cetaceans. Land tortoises are sufficiently protected by their carapace, and therefore have no need of any special modification of structure by means of which their appearance would be assimilated to the surroundings and ' thus give them additional security from their enemies. These, however, are few in number. On the other hand, among the carnivorous terrapins and freshwater turtles instances of protective resemblance are not scarce, and may even attain to a high degree of specialization, as in Chelys, the matamata. The colours of land tortoises are generally plain, or in yellow and brown patterns, whilst those of many terrapins are singularly varied, bright and beautiful, especially in the very young, but all this beauty is lost in the adult of many species.
Chelonians are diurnal animals; only a few are active during the night, habitually or on special occasions, as, for instance, during oviposition. Land tortoises are slow in all their movements, but all kinds living in water can execute rapid motions, either to seize their prey or to escape from danger. All Chelonians are stationary, residing throughout the year in the same locality, with the exception of the marine turtles, which periodically migrate to their breeding-stations. Species inhabiting temperate regions hibernate.
Land tortoises, a few terrapins, and some of the marine turtles are herbivorous, the others carnivorous, their prey consisting chiefly of fish, frogs, molluscs, and other small aquatic animals; some, e.g. Clemmys insculpta and Cistudo caroling, have a mixed vegetable and animal diet.
All Chelonians are oviparous, and the eggs are generally covered with a hard shell, mostly elliptical, rarely quite round, as in the case of the marine turtles. The various modifications, and also the not uncommon individual variations, in the composition of the carapace plates and the number and disposition of the shields, are very significant. They show an unmistakable tendency towards reduction in numbers, a concentration and simplification of the shell and its covering shields. We can to a certain extent reconstruct a generalized ancestral tortoise and thereby narrow the wide gap which separates. the Chelonia from every other reptilian order. The early Chelonians possessed most likely more than five longitudinal dorsal rows of plates. The presence of several small supramarginal shields in Macroclemmys may be an indication that the total number of longitudinal rows was originally at least seven. The number of transverse rows, both of plates and shields, was also greater. We can account for at least twelve median plates and as many pairs of marginals, but for onl y ei g ht median and eight pairs of costal shields (individual variations observed in Thalassochelys). It stands to reason that originally each trunk metamere had its full complement of plates and shields; consequently that about twelve trunk metameres partook in the formation of the shell, which, with subsequent shortening and broadening of the trunk, has undergone considerable concentration and reduction, a process which has reduced the costal plates to seven pairs in the American species of Trionyx, has completely abolished the neural plates of some Chelydidae, and has brought down the costal shields to four pairs in the majority of recent Chelonians. In several species of Testudo the little nuchal shield is suppressed, thereby reducing the unpaired median shields to five. The complete absence of shields in the Trionychidae and in Carettochelys is also due to a secondary process, which, however, has proceeded in a different way.
Classification of Chelonia. H. Stannius in 1854 clearly separated the Trionychoidea from the rest. E. D. Cope, in 1870, distinguished between Pleurodira and Cryptodira according to whether the neck, SEpri or 5apr l, is bent sidewards, or hidden by being withdrawn in an S-shaped curve in a vertical plane; he also separated Sphargis as Athecae from all the other Chelonians, for which L. Dollo, in 1886, proposed the term Thecophora. These terms are most unfortunate, misleading. Athecae (from 817rcrl, shell) has reference to the absence of a horny shell-covering in the leathery turtle; but since the same character applies to Trionychoidea and to Carettochelys, nobody can guess that FIGS. I, 2. - Shell of the integument, which osseous carapace, these Upper or dorsal aspect.
Epidermal shields co, Costals.
pg, Postgulars or humerals. P Pectorals.
pa, Preanals or femorals. an, Anals.
the term Athecae in Dollo's sense refers to the fact that the shell of the leathery turtle is not homologous with the typical shell or NKr) of the other Chelonians. The grouping of the latter into families recognizable by chiefly internal, skeletal characters has been effected by G. A. Boulenger. For practical purposes the following "key" is preferable to those taxonomic characters which are mentioned in the descriptions of the different families. The relationships between them may be indicated as follows: Sub-order I. Athecae. - The shell consists of a mosaic of numerous small polygonal osseous plates and is covered with leathery skin without any horny shields. The limbs are transformed into paddles, without claws. Marine. Sole representative Sphargis or Dermatochelys coriacea, the leathery turtle or luth; it is the largest of living Chelonians, surpassing 6 ft. in length, has a wide distribution over all the intertropical seas, but is very rare everywhere; a few stragglers have appeared as far north as the coasts of Long Island, and those of Great Britain, Holland and France. It is a curious fact that only adults and young, but none of intermediate size, happen to be known. This creature shows many important features. The vertebrae and ribs are not fused with, but remain free from, the carapace, and this is fundamentally different from and not homologous with that of other Chelonians. O. P. Hay has suggested that the mosaic polygonal components of the shell of Sphargis are, so to speak, an earlier generation of osteo of the Thecophora, which in them fuse with the neural arches and the ribs. Sphargis has, howFIG. 4. - The Snapping Turtle (Chelydra serpentina). ever, the later category in the plastron and in its first neural or nuchal of 3 ft. It is characterized by the three series of strong prominent plate. If this suggestion is correct, this turtle has either lost or keels along the back; it inhabits the whole basin of the Mississippi perhaps never had developed the horny shields. The many mosaic and Missouri rivers.
plates comprise larger plates which form an unpaired median, Family 2. Dermatemydidae. - The pectoral shields are widely two pairs of other dorsal, a lateral and three pairs of ventral series separated from the marginals by inframarginals, the gulars are or ridges; thirteen, or when the inner ventral pair fuses, twelve pairs small or absent, and the tail is extremely short. Only a few species, in all. in Central America. The plastron is composed of nine plates. The skull, excellently studied by J. F. van Bemmelen, much The nuchal plate has a pair of rib-like processes like those of the resembles that of Chelone, but so-called epipterygoids are absent; Chelydridae. One or more of the posterior costal plates meet in further, the pterygoids, instead of sending lateral arms to the jugals the middle line. The shell of these aquatic, broadly web-fingered and maxillaries, are widely separated from these bones by the tortoises, is very flat and the covering shields are thin. They feed (Athecae Chelonia I Thecophora J Pleurodira Cryptodira Trionychoidea Key to the Families of Chelonia. Shell covered with horny shields.
Digits distinct, with five or four claws. Pectoral shields separated from the marginals by inframarginals.
Tail long and crested. Plastron small and cruciform Tail long, covered with rings of shields Plastron large Tail short Pectoral shields in contact with the marginals.
Plastral shields II or 12, without an intergular.
Neck retractile in an S-shaped vertical curve Plastral shields 13, an intergular being present.
Neck bending sideways under the shell Limbs paddle-shaped, with one or two claws Shell without horny shields, covered with soft leathery skin.
Digits distinct, broadly webbed, but with only three claws Limbs paddle-shaped.
Shell composed of regular series of bony plates. Two claws Shell composed of very many small plates arranged like mosaic. No claws .
Sphargidae Pelomedusidae Chelydidae Carettochelydidae Chelydridae - Dermatemydidae - Cinosternida Platysternidae Testudinidae Chelonidae Chelydridae Platysternidae Dermatemydidae Cinosternidae Testudinidae ? Chelydidae Pelomedusidae Chelonidae Trionychoidea Carettochelydidae Sphargidae.
palatines, and these do not at all ventrally roof over the choanae. The position of Sphargis in the system is still a moot question. G. A. Boulenger looks upon it as the sole remnant of a primitive group in opposition to all the other recent Chelonia; G. Baur considered it the most specialized descendant of the Chelonidae, a FIG. 3. - A portion of the Osseous Plates of the Carapace of Sphargis coriacea, showing three large keeled plates of one of the longitudinal ridges of the carapace, with a number of the small irregular plates on either side of them.
view which has been supported by W. Dames, E. C. Case, and to a certain extent by J. F. van Bemmelen. For literature, &c., see L. Dollo, Bull. S. R. Bruxelles (Fevrier 4, 1901).
Sub-order II. Thecophora. - The bony shell is composed of several longitudinal series of plates (on the dorsal side a median or neural, a paired lateral or costal series, and marginal plates). With few exceptions this shell is covered with large horny scutes or shields.
Super-family 1. Cryptodira. - The neck, if retractile, bends in an S-shaped curve in a vertical plane. The pelvis is not fused with the shell, and this is covered with large horny shields, except in Carettochelys. Family 1. Chelydridae. - The plastron is rather narrow, and crossshaped; the bridge is very narrow and is covered by a pair of shields, the displaced abdominals, which are separated from the marginals by a few inframarginals. The limbs, neck and head are so stout that they cannot completely be withdrawn into the shell. The tail is very long. Only two genera with three species, confined to America. Chelydra serpentina, the snapping turtle,"ranging from the Canadian lakes through the United States east of the Rockies; closely allied is C. rossignoni of Central America and Ecuador. Macroclemmys temmincki, the" alligator turtle,"is the largest known fresh-water Chelonian, its shell growing to a length dermal plates than the fewer and larger plates upon leaves, grass and especially fruit. Staurotypus, e.g. salvini with 23, Dermatemys, e.g. mawi, with 25 marginal shields.
Family 3. Cinosternidae. - Closely allied to the two previous families from which Cinosternum, the only genus, differs chiefly by the absence of the endo-plastral plate. Inframarginals are present. The nuchal plate has a pair of rib-like processes. The neural plates are interrupted by the meeting of several pairs of the costal plates. Twenty-three marginal shields. In some species the skin of the legs and neck is so baggy that these parts slip in, the skin rolling off, when such a turtle withdraws into its shell. In some the plastron is hinged and the creature can shut itself up tightly, e.g. C. leucostoma of Mexico; in others the plastron leaves gaps, or it is narrow and without hinges, C. odoratum, the mud turtle or stinkpot terrapin of the eastern half of North America. About a dozen species, mostly Central American.
Family 4. Platysternidae. - Platysternum megacephalum, the only species, from Burma to southern China. The total length of these thick-headed, very long-tailed turtles is about i ft., only 5 in. belonging to the shell. The plastron is large, oblong, not cruciform, composed of nine plates. The nuchal is devoid of rib-like processes. A unique arrangement is that the jugals are completely shut off from the orbits owing to the meeting of the post-frontals with the maxillaries.
Family 5. Testudinidae. - The shell is always covered with welldeveloped shields; those which cover the plastral bridge are in direct contact with the marginals. The plastron is composed of nine bones. The digits have four or five claws. The neck is completely retractile.
This family contains the majority of tortoises, divided into as many as 20 genera. These, starting with Emys as the least specialized, can be arranged in two main diverging lines, one culminating in the thoroughly aquatic Batagur, the other in the exclusively terrestrial forms. Emys, with the plastron movably united to the carapace; with well-webbed limbs, amphibious. E. orbicularis or europaea was, towards the end of the Pleistocene period, distributed over a great part of middle Europe, remains occurring in the peat of England, Belgium, Denmark and Sweden; it is now withdrawing eastwards, being restricted in Germany to isolated localities east of Berlin, but it reoccurs in Poland and Russia, whence it extends into western Asia; it is common in south Europe. The other species, E. blandingi, lives in Canada and the north-eastern states of the Union. Clemmys with the plastron immovably united to the carapace; temperate holarctic region, e.g. C. caspica, C. leprosa in Spain and Morocco; C. insculpta, in north-east America. Malacoclemmys with a few species in North America, e.g. M. terrapin, the much prized" diamond-back. "Chrysemys with many American species, e.g. Ch.. pitta, the" painted terrapin "and C. concinna, most of them very handsomely coloured and marked when still young. Batagur and Kachuga in the Indian sub-region.
Cisttsdo carolin g , the box tortoise of North America, with the plastron divided into an anterior and a posterior movable lobe, so that the creature can shut itself up completely. Although essentially by its internal structure a water tortoise, it has become absolutely terrestrial in habits, and herewith agree the highbacked instead of depressed shell, the short webless fingers and its general coloration. It has a mixed diet. The eyes of the males are red, those of the females are brown. From Long Island to Mexico. Cinixys, e.g. belliana of tropical Africa, has the posterior portion of the carapace movably hinged. Pyxis arachnoides of Madagascar has the front-lobe of the plastron hinged.
Testudo, the main genus, with about 40 species, is cosmopolitan in tropical and sub-tropical countries, with the exception of the whole of the Australian and Malay countries; most of the species are African. T. graeca, in Mediterranean countries and islands. T. marginata in Greece with the posterior margin of the carapace much flanged or serrated, and T. ibera or mauritanica from Morocco to Persia; both differ from T. graeca by an unpaired supracaudal, marginal shiold, and by the possession of a strong, conical, horny tubercle on the hinder surface of the thigh. With age the posterior portion of the plastron develops a transverse ligamentous hinge.
T. polyphemus, the" gopher "of southern United States, lives in pairs in self-dug burrows. T. tabulata is one of the few South American terrestrial tortoises.
Of great interest are the so-called gigantic land tortoises. In former epochs truly gigantic species of the genus Testudo had a wide and probably more continuous distribution. There was T. atlas, of the Pliocene of the Sivalik hills with a skull nearly 8 in. long, but the shell probably measured not more than 6 ft. in length, the restored specimen in the Natural History Museum at South Kensington being exaggerated. T. perpigniana of Pliocene France was also large. Large land tortoises, with a length of shell of more than 2 ft., became restricted to two widely separated regions of the world, viz. the Galapagos Islands (called thus after the Spanish galapago, i.e. tortoise), and islands in the western Indian Ocean viz. the Mascarenes (Bourbon, Mauritius and Rodriguez) and Aldabra. When they became extinct in Madagascar is not known, but T. grandidieri was a very large kind, of apparently very recent date. At the time of their discovery those smaller islands were uninhabited by man or any predaceous mammal. It was on these peaceful islands that land tortoises lived in great numbers; with plenty of food there was nothing for them to do but to feed, to propagate, to grow and to vary. Most of the islands were or are inhabited by one or more typical, local forms. As they provided, like the equally ill-fated dodo and solitaire, a welcome provision of excellent meat, ships carried them about, to be slaughtered as occasion required, and soon almost exterminated them; some were occasionally liberated on other islands, for instance, on the Seychelles and on the Chagos, or they were left as presents, in Ceylon, Java or on Rotuma near the Fijis. Thus it has come to pass that the few survivors have been very much scattered. The small genuine stock at Aldabra is now under government protection, in a wa y. A large male of T. gigantea or elephantina or hololissa or ponderosa, was brought to London and weighed 870 lb; another specimen had in 1908 been living at St Helena for more than one hundred years. A specimen of T. daudini, native of the South Island of Aldabra, was known for many years on Egmont Island, one of the Chagos group, then it was taken to Mauritius and then to England, where of course it soon died; its shell measures 55 in. in a straight line, and it weighed 560 lb. The type specimen of T. sumeirei, supposed to have come originally from the Seychelles, was in 1908 still kept in the barrack grounds at Port Louis, Mauritius, and had been known as a large tortoise for about 150 years.
T. vosmaeri was a very thin-shelled species in Rodriguez. Of the Galapagos species T. ephippium still survives on Duncan Island; T. abingdoni lived on Abingdon Island; of T. elephantopus or vicina, G. Baur still collected 21 specimens in 1893 on Albemarle Island. One monster of this kind is said to have measured 56 in. over the curve of the carapace, with a skull a little more than 7 in. in length. All the Galapagos species are remarkable for their comparatively small head and the very long neck, which is much larger and more slender than that of the eastern species.
Family 6. Chelonidae. Marine turtles, with only two recent genera, with three widely distributed species. The limbs are paddleshaped, with only one or two claws, and the shell is covered with horny shields. The neck is short and incompletely retractile. The parietals, post-frontals, squamosals, quadrato-jugals, and jugals are much expanded and form an additional or false roof over the temporal region of the skull.
The Chelonidae are a highly specialized offshoot of the Cryptodira, adapted to marine life. Fundamentally they agree most with the Testudinidae, and there is nothing primitive about them except that they still possess complete series of inframarginal shields.
Chelone, with only 4 pairs of costal shields, with 5 neurals and a broad nuchal. C. mydas s. viridis, the" green or edible turtle,"FIG. 5. - Green Turtle (Chelone mydas). has, when adult, a nearly smooth shell. It attains a length of nearly 4 ft., and may then weigh more than three hundredweight. Their food consists of algae, and of Zostera marina. Their capture forms a regular pursuit wherever they occur in any numbers. Comparatively few are caught in the open sea, others in staked nets, but the majority are intercepted at well-known periods and localities where they go ashore to deposit their eggs. These are round, with a parchment-like shell and buried in the sand, above the high-tide mark, as many as 100 to 250 being laid by one female. They are eagerly searched for and eaten. The famous turtlesoup is made not only of the meat and the fat, but also from the thick and gelatinous layer of subcutaneous tissue which lines the inside of the shell. Only the females are eaten; the males, recognizable by the longer tail, are rejected at the London market. This species inhabits the Atlantic, Indian and Pacific Oceans.
C. imbricate, the" hawksbill turtle. "The shields are thick, strongly overlapping each other from before backwards, but in old specimens the shields lose their keel, flatten and become juxtaposed. The horny cover of the upper jaw forms a hooked beak. This species lives upon fish and molluscs and is not eaten; but is much persecuted for the horny shields which yield the tortoise-shell," so far as this is not a fraudulent imitation. When heated in oil, or boiled, the shields (which singly are not thick enough to be manufactured into larger articles) can be welded together under pressure and be given any desired shape. The "hawksbill" FIG. 6. - Hawksbill Turtle (Chelone imbricata). ranges over all the tropical and sub-tropical seas and scarcely reaches 3 ft. in length, but such a shell yields up to 8 lb of tortoiseshell.
Thalassochelys caretta, the "loggerhead," has normally five pairs of costal shields, but whilst the number' of shields in the genus Chelone is very constant, that of the loggerhead varies individually to an astonishing extent. The greatest number of neurals ob. served, and counting the nuchal as the first, is 8, and 8 pairs of costal, in all 24; the lowest numbers are 6 neurals with 5 pairs of costals; odd costals are frequent. The most interesting facts are that some of the supernumerary shields are much smaller than the others, sometimes mere vestiges in all stages of gradual suppression, and that the abnormalities are much more common in babies and small specimens than in adults. The importance of these orthogenetic variations has been discussed by H. Gadow in A. Willey's Zoolog. Results, pt. iii. p. 207-222, pls. 24, 25 (Cambridge 1899).
FIG. 7. - Loggerhead (Thalassochelys caretta). The "loggerhead" is carnivorous, feeding on fish, molluscs and crustaceans, and is not esteemed as food. A great part of the turtle-oil which finds its way into the market is obtained from it; its tortoiseshell is of an inferior quality. Besides all the intertropical seas it inhabits the Mediterranean, and is an accidental visitor of the western coasts of Europe. The old specimen captured on the Dutch coast in 1894 contained the enormous number of 1150 eggs.
Super-family 2. Pleurodira. - The long neck bends laterally and is tucked away between the anterior portion of the carapace and the plastron. The dorsal and ventral ends of the pelvis are anchylosed to the shell. Fresh-water tortoises of South America, Australia, Africa and Madagascar.
FIG. 8. - The Matamata (Chelys fimbriata) with side view of head, and separate view of plastron.
Family I. Pelomedusidae. - Neck completely retractile. Carapace covered with horny shields, of which the nuchal is wanting. Plastron composed of II plates. With 24 marginal and 13 plastral shields, FIG. 9. - Lower view of Trionyx euphratica. inclusive of a conspicuous intergular. Sternothaerus in Africa and Madagascar. Pelomedusa galeata in Madagascar and from the Cape to the Sinaitic peninsula. Podocnemis is common in tropical South America, e.g. P. expansa of Brazilian rivers, noteworthy for the millions of eggs which are, or were, annually collected for the sake of their oil. Bates (The Naturalist on the River Amazon) gives a most interesting account of these turtles, which are entirely frugivorous.
Family 2. Chelydidae. - The neck, when bent, remains partly exposed. Shell covered with shields. Plastron composed of 9 plates; but covered with 13 shields. This family, still represented by nearly 30 species, with 8 genera, is found in South America and in Australia. Chelys fimbriata, the "matamata" in the rivers of Guiana and North Brazil; total length about 3 ft.; with animal diet. Hydromedusa, e.g. tectifera, with very long neck, in Brazil, much resembling Chelodina, e.g. longicollis of the Australian region.
Family 3. Carettochelydidae. - Carettochelys insculpta, the only species, in the Fly river of New Guinea; still imperfectly known. This peculiar turtle seems to stand in the same relation to the Chelydidae and to the Trionychidae as do the Chelonidae to the Testudinidae by the transformation of the limbs into paddles with only two claws, and the complete reduction of the horny shields upon the shell, which is covered with soft skin. The plastron is composed of 9 plates; the 6 neural plates are all separated from one another by the costals. The premaxilla is single, as elsewhere only in FIG. Io. - Upper view of the Turtle of the Euphrates (Trionyx euphratica). Chelys and in the Trionychidae. The neck is short and non-retractile. Length of shell about 18 in.
Super-family 3. Trionychoidea. - The shell is very flat and much smaller than the body, and covered with soft leathery skin, but traces of horny structures are still represented, especially in the young of some species, by numerous scattered little spikes on the back of the shell and even on the soft parts of the back. The limbs are short, broadly webbed and only the three inner digits are provided with claws. Head and neck are retractile, bending in a sigmoid curve in a vertical plane. The jaws are concealed by soft lip-like flaps and the nose forms a short soft proboscis. The temporal region is not covered in by any arches; the quadrate is trumpetshaped as in the Chelydidae, but the jugular arch is complete. The pelvis is not anchylosed to the shell. The carapace is much reduced in size, the ribs extending beyond the costal plates, and there are no marginals; except in the African Cyclanorbis the neural plates form a continuous series. All the nine elements of the plastron are deficient and but very loosely connected with each other. Most of these reductions in the skeletal and tegumentary armature are the result of life in muddy waters, in the bottom of which these creatures bury themselves with only the head exposed. They feed upon aquatic animals; those which are partial to hardshelled molluscs soon wear down the sharp horny edges of the jaws, and thick horny crushing pads are developed in their stead. They only crawl upon land in order to lay their round brittle eggs. Trionyxes inhabit the rivers of Asia, Africa and North America. Trionyx ferox, the "soft-shelled turtle," in the whole of the Mississippi basin and in the chain of the great northern lakes. T. triunguis in Africa, the largest species, with a length of shell of 3 ft. T. hurum and T. gangeticus are the commonest Indian species. The young are ornamented with two or three pairs of large, round, ocellated spots on the back. (H. F. G.)
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Chisholm, Hugh, General Editor. Entry for 'Tortoise'. 1911 Encyclopedia Britanica. https://www.studylight.org/encyclopedias/eng/bri/t/tortoise.html. 1910.