STERNUM 



53 



ICHTHYOPSIDA.— Several sharks (fig. 56, A and B) have a 

 medial cartilage between the coracoidal parts of the pectoral girdle 

 which, apparently is homologous with the mammalian presternum, 

 but its development is unknown. A similar structure is known as 

 high as the reptiles and Dipnoi, but none is known in Teleosts, while 

 in many Urodeles a sternal cartilage occupies the same position (fig. 

 57) its anterior margins being grooved to receive the postero-lateral 



Fig. 56. — Parts of the pectoral girdle of (.4), Acanthias; B. Hexanlhus (White) and 

 C, Nolhosaurus (Zittel, '92). c, co, coracoid; cU clavicle; p, presternum. 



borders of the coracoids. Some Urodeles lack all sternal structures, 

 and none are found in Gymnophiona or in the Stegocephals, the 

 episternum of the latter group being a membrane bone of the pectoral 

 girdle. 



Fig. 57. — Sternum and pectoral girdle 

 oi Siren (Cope, '89). c, coracoid; h, hume- 

 rus; p, precoracoid; 5, scapula (bent out- 

 wards); si, sternum; cartilage stippled, 

 bone black. 



Fig. 58. — Pectoral girdle and sternum 

 of " Calamites" (Parker, '68). cl. clavicle; 

 CO, coracoid; e, epicoracoid; g, glenoid fossa; 

 o, omosternum; pc, precoracoid; s, scapula; 

 St, sternum. 



The lower Anura (Arcifera) resemble the Urodeles, there being a 

 presternum which embraces the median parts of the coracoids and 

 sometimes (some Hyhds, fig. 58) is prolonged backwards as a pair of 

 horns which recall the mesosternal bars of the developing mammal. 

 In the higher Anura (Firmisterna), the epicoracoids of the two sides 

 meet in the middle hne and sternal structures occur in front of and 

 behind the girdle, (fig. 58). The anterior of these is commonly 



