CHAPTER IX 



CHELONIA ATHECAK TIIECOPHORA 



Sub-Class IV.— CHELONIA. 



There is no mistakint>; a tortoise. The shell and the horn- 

 covered toothless jaws separate them from . all other four-footed 

 creatures. . 



They may be described as terrestrial or aquatic, pentadactyle 

 reptiles, with walking limbs or with paddles ; ribs with capitular 

 portions only, two sacral vertebrae, humerus with entepicondylar 

 foramen, pubes and ischia forming symphyses, quadrate bones 

 fixed, jaws without teeth, but with cutting horny sheaths. 

 Trunk encased in a bony shell, composed of numerous dorsal and 

 ventral dermal bones, forming a carapace and a plastron, which 

 may or may not be covered with horny shields. Copulatory 

 organ unpaired, cloacal opening more longitiidinal than round, 

 never transverse. Oviparous. 



It is customary to distinguish the marine, paddle-limbed kinds 

 as Turtles, the others as Land- and Water-tortoises. 



Tortoises occur already in the Trias. They reached their 

 greatest development towards the end of the Mesozoic and in 

 the earlier Tertiary periods. They are now comparatively 

 reduced in the number of families and genera, although they 

 are still represented by about 200 species. The sub-class as a 

 whole is cosmopolitan, but does not occur in the colder regions. 



Their t)rigin is (juilc inikiiDwii. Of recent groups unly the 

 Crocodilia and the lihynchocephalia come into consideration. 

 Combination of these groups with the Chelonia leads to some 

 unknown forms whence also the Theromorpha have arisen. 

 I'alaeontology does not help us, all the leading, main groups of 

 Chelonia haviny; been in existence in the earlier jMesozoic ages , 



