194 THE VERTEBRATE SKELETON. 



head and is partially attached to two vertebrae ; there are no 

 cervical or sternal ribs. There is no true sternum. 



The three anterior elements of the plastron are respectively 

 homologous with the interclavicle and two clavicles of other 

 reptiles, while the remaining elements of the plastron are pro- 

 bably homologous with the abdominal ribs of Crocodiles. The 

 pectoral girdle lies within the ribs, and the precoracoids and 

 coracoids do not meet in ventral symphyses. The scapula and 

 precoracoid are ossified continuously. The pubis probably 

 corresponds with the prepubis of Dinosaurs. There are four 

 limbs each with five digits. 



The order includes three suborders : 



Suborder (1). TRIONYCHIA. 



The carapace and plastron have a rough granular surface 

 covered with skin and without any horny shields. 



The plastron is imperfectly ossified, and marginal bones 

 may be absent, or if present are confined to the posterior 

 portion of the carapace. The pelvis is not united to the 

 plastron. The cranium has not a complete false roof and the 

 head can be drawn back under the carapace. 



The first three digits of both manus and pes bear claws, 

 and the fourth digit in each case has more than three pha- 

 langes. The most important genus is Tribnyx. 



Suborder (2). CRYPTODIRA. 



The carapace and plastron vary in the extent to which they 

 are ossified, and except in Dermochelys 1 and its allies are 

 covered by horny plates. Marginal bones are always present. 

 The head can generally be drawn back under the carapace. 

 The pelvis is not firmly united to the plastron. The cranium 

 often has a complete false roof, and in the mandibular articu- 

 lation the cup is borne by the cranium, and the knob by the 

 mandible. Among the more important genera are Dermo- 

 chelys, Ckelone, and Testudo. 



1 See p. 272. 



