io6 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



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, a second 

 bone, the cleithrum. In some ganoids (Polypterus, fig. no) 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 consequently 



FIG. no. Pectoral girdles of (A) Acipenser and (B} Polypterus, after Gegenbaur. ct, 

 cleithrum; cv, clavicula; dr, dermal rays; g, glenoid surface. 



disappeared. Dorsal to the cleithra other membrane bones frequently 

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

 temporals, fig. 79) 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. 58). The bone 

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

 slender bone, usually called scapula, but by some the cleithrum. A 



