XII 



PHYLUM CHORDATA 



413 



a rounded plate of cartilage, the omosternum ; and passing 

 backwards from their posterior ends is a similar but larger 

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

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

 The four limbs deviate from the typical structure (p. 339) 

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

 radio-ulna (Fig. 231, 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 

 complete digits are the second to 

 the fifth of the typical hand. Six 

 carpals only are present. 



The pelvic arch (Figs. 231 and 234) 

 is very peculiarly modified ; it re- 

 sembles in form a Bird's " merry- 

 thought," consisting of two long 

 curved bars articulating in front with 

 the transverse processes of the sacral 

 vertebra and uniting posteriorly in 

 an irregular vertical disc of mingled 

 bone and cartilage which bears on 

 each side a deep, hemispherical 

 acetabulum (Fig. 234, G) for the ar- 

 ticulation of the thigh-bone. The 

 curved rods are the ilia (//., P) 

 they expand posteriorly and unite 



with one another in the median plane to form the dorsal 

 portion of the disc and about one-half of the aceta- 

 bulum. The posterior portions of the disc and the 

 acetabulum are furnished by the ischia (Is.\ fused with one 

 another in the sagittal plane, and their ventral portions by 

 the similarly united pubes (Kn\ The ilium and ischium are 

 formed of true bone, the pubis of calcified cartilage ; the 



FIG. 234. Rana esculenta. 

 Pelvic girdle from the right 

 side. G. acetabulum ; //,/*. 

 ilium ; Is. ischium ; A'n. 

 pubis (FromWiedersheim's 

 Comparative Anatomy.) 



