Xii PHYLUM CHORDATA 415 



Passing forwards from the anterior ends of the united 

 epicoracoids is a rod of bone, the episternum (Ep), tipped 

 by a rounded plate of cartilage, the omosternum ; and passing 

 backwards from their posterior ends is a similar but larger 

 bony rod, the sternum (Si), also tipped by a cartilaginous 

 plate, to which the name xiphisternum (Jn) is applied. 



JSjv 



FlG. 249. Rana CSCUlenta. The shoulder girdle from the ventral aspect. Co, 

 coracoid; Co', epicoracoid; Cl, clavicle; G, glenoid cavity; Ep, episternum; 

 Fe, fenestra between procpracoid and coracoid; KC, cartilage separating scapula 

 and clavicle; Kn, xiphisternum; m, junction of epicoracoids; S, scapula; 

 St, sternum. (From Wiedersheim's Comparative Anatomy.) 



The four limbs deviate from the typical structure (p. 341) 

 chiefly in the fusion of the radius and ulna into a single 

 radio-ulna (Fig.' 247, RA. UL) and in the presence of only 

 four complete digits with a vestigial one on the radial side. 

 In all probability the latter represents the pollex, and the 



