VERTEBRATES. 



23 



starting point. In all of the higher forms we no longer meet with an undivided arch, 

 hut the point of attachment of the limb (glenoid fossa on the pectoral, acetabulum on 

 the pelvic girdle) divides the cartilage 

 into upper and lower regions (Figs. 28, 

 29). The upper is styled scapula in the 

 pectoral, ilium in the pelvic girdle. While 

 the dorsal end of the scapula is uncon- 

 nected with the vertebral column, the 

 ilium acquires such a connection by the 

 intervention of one or more pairs of sac- 

 ral ribs. Although the number of sacral 

 vertebrae so formed is primarily small, 

 the ilium may coalesce with vertebra; in 

 front of and behind them, in such a way 

 as to secure a great amount of solidity in 

 this region, as is the case with the birds. 

 The region of cartilage below the attach- 

 ment of the limb generally ossifies in 

 two 2)ieces, an anterior and posterior, 



between which in all forms, except Am- Fig. 28.— Half of pectoral girdle of turtle ; c, coracoid ; c(, 

 , ., . „ , T T clavicle ; f, epicoracoid ; I/, glenoid fossa ; s, scapula. 



phibia, a fenestra occurs. In the j)ec- 



toral girdle, the bone in front of the fenestra is termed jirecoracoid or clavicle, that 

 behind it, the coracoid ; and these elements correspond respectively to the pubis and 

 ischium of the pelvic girdle, which are similarly related to the fenestra there. Con- 

 necting the elements of either side in 

 the middle line, we find in many forms 

 strips of cartilage (epicoracoid, sym- 

 physial) which may project in front 

 into cartilaginous or osseous processes, 

 styled respectively episternum and 

 ejjipubis. The term episternum is 

 suggested by the fact that in some 

 Amphil>ia as well as in other reptiles 

 and in birds, although not turtles, the 

 coracoids acquire a secondary connec- 

 tion with the sternum, which persists 

 in the monotremes, the mammals 

 which are nearest the reptiles in their 

 organization. 



The condition of these parts in the 

 other reptiles and Amphibia will be 

 readily understood from the foregoing 

 description, but some reference is nec- 

 essary to the modifications characteris- 

 tic of birds and mammals. 



The pectoral arch in birds is characterized by the stoutness of the coracoid 

 element, and the slenderness of the clavicular. The latter is developed partly 

 in membrane, and the clavicles of opposite sides meet in the ventral, middle line, 



Fig. 29.— Pelvis of turtle ; a, acetabulum ; e, epipubis ; il, 

 ilium; is, ischium; pt^ pectineal process; pu, pubis ; sr, 

 sacral ribs ; sv, sacral vertebrae. 



