4 ANATOMY OF A CHELONIAN. 



11. The corpora bigemiiia lie on the dorsal aspect of 

 the brain. The spinal cord presents no sinus rhom- 

 boidalis. 



12. The caudal vertebrae often form a series equal 

 in length to the rest of the body. 



13. Teeth are frequently present. 



Characters of the Chelonia. 



1. None of the vertebrae have transverse processes, 

 and the dorsal vertebrae are not movable on One 

 another ; nor are the ribs movable upon the vertebrae. 

 Most of the dorsal vertebrae, and ribs are restrained from 

 motion by the union of superficial bony plates, into 

 which they pass, to form a carapace. 



2. All but the soft tortoises {Trionyx) have part of 

 the epidermis modified to form horny plates, which 

 constitute the tortoise-shell. These plates are arranged 

 in definite series on the carapace and plastron. 



3i The presacral vertebrae are few. 



4. No sternal ribs, and no trace of a true sternum. 

 The ventral surface is protected by a number (usu- 

 ally nine) of dermal membrane bones, forming the 

 plastron. 



5. The sacrum consists of only two vertebrae ; the 

 sacral ribs are not anchylosed to their vertebrae. 



6. The anterior caudal vertebrae have no transverse 

 processes, but some have ribs. 



7. In the skull, all the bones but the mandible and 

 hyoidean arch are immovably united together: the 

 epiotic anchyloses to the supra-occipital ; the prootic 

 and opisthotic remain as distinct bones. The alisphe- 



