2s2 



VERTEBRATE SKELETON 



sternum (fig. 62) . The scapula, expanded dorsally, is largely ossified 

 and the suprascapula is small. 



Belodon (extinct) has a short rounded coracoid and a single large fenestra; 

 the scapula is moderate. The Mesosuchia have lost the clavicles, while the 

 elongate coracoid contains a supracoracoid foramen. 



Chelonia differ from all other Vertebrates in the shifting of the 

 pectoral girdle back under the dermal armor, a part of which is 

 formed by the ribs. The girdle (fig. 267) consists of three rods, one 

 dorsal and two ventral, all preformed in cartilage and meeting at the 

 glenoid fossa. The interpretations of these differ. The dorsal rod 



is clearly the scapula; it has a 

 small suprascapular cartilage and 

 is connected by ligament or carti- 

 lage with the anterior costal plate 



Fig. 267. — Pectoral girdle and part of 

 plastron of Chelone midas (Parker, '68). 

 c, coracoid; en, entoplastron; usually re- 

 garded as homologue of episternum; ep, 

 epiplastron (less certainly homologue of 

 clavicle); hp, hyoplastron; pc, precora- 

 coid; sc, scapula. 



of the carapace. The anterior 

 ventral bar forms a continuum 

 with the scapula, and when an 

 entoplastron is present, the ventral 

 end of the rod is connected with it 

 by cartilage or ligament. The 

 second bar extends medially and 

 backwards from the fossa; its 

 medial end is expanded, but those 

 of the two sides do not usually meet, although they overlap in 

 Dermochelys. The medial ends of the ventral bars of a side are con- 

 nected by ligament which is sometimes condrified. 



The usual view is that the anterior ventral bar is the precoracoid, 

 the posterior the coracoid, a view which receives confirmation, if 

 the cartilage at times connecting these be the epicoracoid. Those 

 holding this view recognize the episternum in the entoplastron, 

 and often the clavicles in the epiplastra. (On this last point 

 see below.) Another interpretation is that the anterior rod is a 

 greatly developed acromion, no precoracoid being present. Others 

 consider it, as in Anura, composed of both precoracoid and clavicle, 

 although it contains no membrane bone. 



The assumption of homology of clavicle and epiplastron involves phylo- 

 genetic difficulties. Clavicles appear as low as the Ganoids, and it is easier to 

 assume their loss in Chelonia than to suppose they have remained in the skin 



