88 



THE COMMON FROG. 



[chap. 



closely approximated one against the other. The 

 great elongation and small attachments of the ilia 

 allow the pelvis as a whole to be bent upon the back- 

 bone. Thus the hinder part of the body is movable, 

 and forms, as it were, an additional common root seg- 

 ment for the two limbs. 



The skeleton of the ankle as developed in the frog's 

 class presents us with some characters, which, more 

 than even those of the wrist, suggest the passage of 

 the line of affinity directly from Batrachians to mam- 

 mals, leaving both reptiles and birds on one side. 



Fig. 54- — Bones of foot of Frog. — «, astragalus ; c, os calcis ; ac, united portions of 

 these bones ; li, extra ossicle of inner side of foot ; cb, ossicle representing cuboid 

 and other tarsal bones — i, 2, 3, 4, 5 — the five metatarsals. 



In the first place we meet in the frog with certain 

 extra ossicles in the inner side of the foot, which 

 present the appearance of a broad rudiment of an 

 extra digit on the inner side of the great toe. Now 



