82 THE COMMON FROG. [chap. 



order, which has a skull, the temporal fossa of which 

 is similarly enclosed by bony plates. 



Fig. 46. — External form of Lopkiomys. 



This unexpected discovery completely destroys any 

 weight which might be attached to this character as 

 an evidence of genetic affinity. It does so, because it 

 is inconceivable that this Rodent should have directly 

 descended from a common progenitor of frogs and 

 of Chelonians through a line of ancestors which 

 never lost this cranial shield, though the ancestors 

 of all other beasts, all birds, and all reptiles, except 

 turtles, did lose it. It is inconceivable, for if it were 

 true, a variety of the lowest mammals (Marsupials ^ 

 and Monotremes^) must have less diverged from 

 the ancient common stock than have the members 

 of the Rodent order, and nevertheless these lowest 

 mammals exhibit no trace whatever of such a cranial 

 shield. 



Here then we have an undoubted example of the 



• i.e. Opossums, kangaroos, &c. 



" The Omithorhynchus and Echidna. 



