106 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. 110) 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 
Fic. 110.—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 
