ORDER VI GHELONIA 195 



are either widely separated from each other, or in contact and limiting two 

 obturator foramina (Testudmidae). The pubis sends oflf a more or less developed 

 process, directed forwards and outwards, which may be subcylindrical, rod-like, 

 or flat and expanded distally ; a lateral process is also present, but usually less 

 developed than that of the pubis, on the ischium of all Cryptodires except in 

 the Chelonidae (Boulenger). The femur is a curved cylindrical bone, and the 

 tibia and fibula are subequal in length. An astragalus (formed of the tibiale + 

 intermedium) which is in contact with both tibia and fibula, and a small outer 

 calcaneum (fibulare) constitute the proximal tarsals ; or among the Emyds, 

 these may coalesce in a single piece. A centrale is distinct in the Chelydridae 

 and most Pleurodires ; and except in the Chelonidae, which have four, the 

 distal row of tarsals contains five bones. 



Habitat and Geological History. — Many of the Chelonia are terrestrial in habit, 

 a greater number are aquatic, and a few are exclusively marine. About 200 

 recent species are regarded as well established, most of which are limited to 

 the tropics or warm temperate zones. The earliest known fossil remains are 

 from the Upper Keuper of Wiirtemberg (Proganochelys, Chelytherium), and are 

 fortunately tolerably well preserved. None have been discovered in Lower 

 Jurassic rocks, but from the Upper Jura of Solothurn, Northern France, 

 Germany, England, and the United States, a number of forms are known, all 

 of which are closely similar to existing genera. Numerous Chelonian remains 

 occur in the Cretaceous and Tertiary, but complete skeletons in association 

 with the skull are extremely rare. The only noteworthy marks of evolution 

 aflecting the order since early Cretaceous time are degeneration of the carapace 

 and plastron in certain types, and elongation of the phalanges in truly marine 

 forms. 



Baur supposes ancestral Chelonians to have had a habitat similar to that 

 of modern Crocodiles, namely in shallow water or in swamps. A branch then 

 arose which inhabited rivers, and whose most specialised members are the 

 three-clawed mud-turtles (TrionycJioidea). That these forms with soft marginal 

 plates are descended from a group having completely ossified marginals there 

 can be little doubt. From a fluviatile habitat, according to Baur, these early 

 Chelonians passed on to a marine, prior certainly to the Cretaceous period, 

 since we find here such specialised genera as Protostega, Protosphargis, Alio- 

 pleuron, etc. ; and the most specialised of all is the recent Dermochelys. Still 

 another branch acquired amphibian habits, like many of the Emydidae, and in 

 time some of these {Terrapene, Nicoria, etc.) became truly terrestrial. The land 

 tortoises (TestiuUnidae), a highly specialised group, are initiated in the Eocene 

 (Had nanus), and are completely adapted for a terrestrial existence. 



Sub-Order A. TRIONYCHOIDEA. Bonaparte. 



Dorsal vertebrae and ribs fused with the dermal plates to form an incompletely 

 ossified carapace ornamented with coarse vermiculating sculpture ; no epidermal 

 shields. Neck bending by a sigmoid curve in a vertical plane. Skull with descend- 

 ing parieto-pterygoidal processes. Pterygoids broad throughout, separated from each 

 other, the basisphenoid joining the palatines. . Cervical vertebrae without transverse 

 'processes. Sacral and caudal ribs generally attached to well-developed transverse 

 processes of the neural arches. Pelvis free from the carapace and plastron. Limbs 

 modified into paddles, the fourth digit with at least four phalanges, and only the three 



