54 



VERTEBRATE SKELETON 



called an omosternum (Gaupp calls it episternum, although it is 

 cartilage in origin). Behind the coracoids is a median structure, 

 apparently presternal, which has been called xiphisternum. Both 

 pre- and omosterna are partly cartilage, partly ossified. Nothing 

 is known of their development, and it is uncertain^whether^they are 

 ever connected. 



REPTILTA.- — Several fossil groups of reptiles (Phytosaurs, Pseudosuchia, 

 Ichthyosaurs, Sauropterygia,^ and most Dinosaurs and Theromorphs) have left 

 no traces of sterna, and if one were present it must have been cartilage. Che- 

 Ionia, with their external armor, and Ophidia which use their ribs 

 in locomotion, and some limbless hzards have none. A few 

 Theromorphs {e.g. Keirognathus, fig. 59) had a broad presternum 

 and several Dinosaurs have a pair of oval or hatch etshaped 

 ossicles regarded as sternal. Pterosaurs have a broad thin 

 sternal plate between the ventral ends of the coracoids (fig. 60) 



Fig. 59. — Pectoral girdle and sternum of 

 Keirognathus (Seeley, '88). c, coracoid; cl, 

 clavicle; es, episternum; g, glenoid fossa; pc, 

 precoracoid; sc, scapula; st, sternum. 



Fig. 60. — Sternum of Pterodac- 

 |tylus (H. von Meyer, '60). 



which is prolonged in front as a spine, this thicker and continued back on the 

 broader part as a keel on the ventral surface. In other groups where a sternum 

 exists it never has a part in front of the clavicles comparable to the Anuran 

 omosternum. 



The sternum of normal Hzards is usually calcified cartilage and is 

 a broad rhomboid plate to which from two to four ribs are attached. 

 It is sometimes entire, sometimes perforated by one or two fenestras 

 (fig. 61). Behind it is prolonged by a pair of xiphisternal horns, 

 apparently comparable to those of other groups, although they may 

 be connected with one or more pairs of ribs. The anterior border is 

 grooved to receive the coracoids. The sternum of Sphenodon (fig. 

 264) differs httle from that of normal Hzards. 



^Nothosaurus (fig. 56, C) possibly has a presternum between the clavicles; it is 

 usually called an episternum. 



