THE LOWER VERTEBRATA. 



149 



The arm has the same general bony structure as the 

 rabbit's. The humerus is short and thick, with a strong 

 deltoid ridge, but no olecranon fossa. Radius and ulna are 

 short and fused together, though there is clear indication 

 of the double origin at the distal end; there is a slight 

 olecranon process. In the carpus there are two rows of 

 three small bones each. Only four digits are well-developed, 

 the thumb, or pollex, consisting of a small metacarpal only, 

 without phalanges. The digital formula is 02233. 



The pelvic girdle is much less easily understood than the 

 pectoral ; it is, in fact, in a highly specialized condition, 

 i.e. it departs widely from the general plan of pelvic girdles 

 to be found through most of the air-breathing vertebrates. 

 This specialized con- 

 dition is connected 

 with the adaptation of 

 the hind-legs to the 

 leaping habit. The 

 ilia are very long, and 

 lie nearly parallel to 

 one another ; their 

 posterior ends meet in 

 a median symphysis ; 

 the ischia are small 

 and also meet in a 

 median symphysis ; 

 while the pubic region 

 is cartilaginous and 

 also median. Hence 

 the girdle altogether 

 loses its character of 

 a bony case protecting 

 some of the abdominal 

 viscera, and serves 

 solely for the attach- 

 ment of the powerful 

 leg-muscles. Ilium, ischium, and #ie 

 take part in bounding the acetabulum. 



The hind-limb can be readily compared with that of the 

 rabbit, but all its bones show a great elongation. The 



Fig. 74. PELVIC GIRDLE AND Hmo-Leo 

 OF FROO. 



Side view (after Howes.) The dotted portion 

 between ilium and ischium represents the 

 pubic cartilage. 



pubic cartilage all 



