Families Pelomedusidfe and ClielyJidiB. 347 



so long. Baur *, it is true, has recently expressed the correct 

 view ; but the characters pointed out by him are by no means 

 the only ones which separate the two families, as may be seen 

 from the following diagnoses : — 



Pelomedusid^. 



Plastral bones eleven, mesoplastrals being present. A bony temporal 

 arch; quadrato-jugal present ; prsefrontals in contact; no nasals; pala- 

 tines in contact; dentary single. Second cervical vertebra biconvex. 

 Neck completelij retractile tcithin the shell. 



Africa, Madagascar, South America, 



A. No bony temporal roof ; mesoplastra extending right across the 



plastron : Sternofhcerus. 



B. No bony temporal roof; mesoplastra small and lateral : Pelo- 



medusn. 



C. A bony temporal roof, the quadrato-j ugal forming a suture with 



the parietal ; mesoplastra small and lateral : Podocnemis {=Du- 

 merilid) and Peltocephalus. 



CHELYDIDiE. 



Plastral bones nine. No bony temporal arch, the quadrato-j ugal 

 being absent ; prfefrontals separated throughout ; nasals present, except 

 in Chelys ; palatines separated by the vomer ; dentary bones distinct t. 

 Fifth and eighth cervical vertebrre biconvex. Neck bending under the 

 margin of the carapace, ahvays exposed. 



South America, Australia, and Papuasia. 



A. Neck longer than the dorsal vertebral column : Chelys, Hydro- 



medusa, Chelodina. 



B. Neck shorter than the dorsal vertebral column : Hydraspis, 



Platemys, Elseya, Chelymys. 



It will be observed that, owing to the structure of the Pelo- 

 medusidse, the term Cryptodiran, as opposed to that of 

 Pleurodiran, no longer expresses a distinguishing character. 

 The Peloraedusida3 are in fact " Cryptodiran," and the Chely- 

 dida3 " Phanerodiran." The term " Orthodira " as opposed 

 to that of Pleurodira would convey the correct idea. But I 

 do not suggest such an alteration, for it seems to me the 

 names of higher groups should, whenever practicable, be 

 retained in virtue of the law of priority, like those of genera 

 and species. 



* Zool. Anz. 1887, p. 101. 



t The symphysial suture disappears in adult specimens of Elseya and 

 Chelyynys, but is perfectly distinct in younger specimens of these genera. 



