ON THE OKIGIN^ OF GENERA. 



97 



Comparmg the genera in a general physiological sense, we may 

 parallelize further. 



Aquatic, with digital dilatations. 



Heteroglossa, 



Arboreal : cranium hy- ) -r, , , ^ 



perostosed. \ ^oljved^tes. 



" cranium free. Rhacophorus. 



Acris. 



Trachycephalus. 



j Hyla. 



( Agalychnis. 



The same kind of parallels exists between the primary groups 

 of the Testudinata, as follows : 



Crtptodira. Plefrodira. 



Five complete pairs of bones across the plastron. 



Pleurosternidae. Stemothseridae. 



Four pairs of bones across plastron ; not more than two phalanges on all toes. 



Testudinidae. Pelomedusidae. 



Three phalanges on most digits ; 

 Zygomatic arch ; no parieto-mastoid. 



Eraydidae. Podocneraididae, 



Temporal fossa over-roofed hj parietal. 



Macrochelys. Podocnemis. 



No zygoma ; a parieto-mastoid arch. 



Hydraspididae. 



* 



* 



* 



If we compare the peculiarities of generic structure merely 

 with reference to their adaptation to the animals' habits, we will 

 see the following : 



Cryptodira. 

 Feet reduced for terrestrial progress. 



Testudinidae. 

 Feet normal. 



Anterior lobe of sternum movable. 



Cistudo. 

 Cinosternum. 

 Anterior lobe fixed. 

 Neck very elongate. 



Trionychidae. 

 Neck shorter ; aquatic. 

 Temporal fossa open. 



Emydidae in gen. 

 Temporal fossa over-roofed. 



Cheloniidae. 



Pleurodira. 

 Pelomedusidae. 



Sternothaerus. 



Chelodina. 



Hydraspididae. 

 Podocnemis. 



The parallels between the genera of the American Iguanidae 

 and the Old World Agamidae are similarly quite close. They are 

 shown on Plates II and II a. 

 1 



