SKELETON 



115 



laginous girdle is reinforced by membrane bones derived from the 

 skin. Of these there are at least two on either side, a pair of clavicles 

 which overlie the coracoid region and meet in the middle line, and 

 lateral to each clavicle and extending to or above the glenoid fossa, 

 is a second bone, the cleithnim. In some ganoids (Polypterus, 

 fig. 121) the cleithra extend toward the middle line, and a little 

 higher in the scale, meet and take the strains. This assumption of 

 stress by the membrane bones results, in the higher forms, in the 

 separation of the two halves of the cartilaginous girdle. 



In the higher ganoids and teleosts the cleithrum has increased 

 greatly, usurping the function of the clavicles, which have conse- 



^J^f^f^ 



*juy. 



Fig. 121. — Pectoral girdles of {A) Acipenser and (B) Polypterus, after Gegenbaur. ct, 

 cleithrum; cv, davicula; dr, dermal rays; g, glenoid surface; r, cartilaginous radialia. 



quently disappeared. Dorsal to the cleithra other membrane bones 

 frequently occur. There may be one or two supracleithra (post- 

 er supratemporals, fig. 88) which connect the girdle with the skull, 

 and occasionally others as postclavicle, infraclavicle, etc. As a 

 result of the great development of the cleithra the cartilaginous 

 girdle has been reduced, but it usually has at least two ossifications 

 on either side, a scapula dorsal to the glenoid fossa and a coracoid 

 in the ventral region, these contributing to the support of the 

 appendage. 



AMPHIBIA. — In the stegocephals the cartilage has not been 

 preserved and the bones are variously interpreted (fig. 66). The 

 bone meeting the episternum is the clavicle, and lateral to this is an 

 equally slender bone, usually called scapula, but by some the 



