SKELETON. 



149 



ployed as a means of dividing birds into Ratitae and Carinatae. 

 It is interesting to find a keel existing in the bats and in the 

 fossil pterodactyls. In the mammals the sternum is more 

 elongate, and more ribs contribute to its formation than in the 

 sauropsida. It may consist of as many separate sternebrae as 

 there are ribs connected with it, or these may so unite that but 

 three separate bones can be recognized, a manubrium in front, 

 a body in the middle, and an ensiform process (xiphisternum) 

 behind, the latter extending behind the ribs. 



Fig. 157. Ster- 

 num of dog, show- 

 ing sternebroe. 



Fig. 158. Shoulder girdle of Ornithorhynchus. 

 C, clavicle; CO, coracoid; E, episternum; £C, epi- 

 coracoid; S, scapula; ST, sternum; R, ribs. 



Connected with the sternum in many groups is a structure 

 to which the name episternum has been given. This first appears 

 in the stegocephali, but reaches its highest development in the 

 reptiles. It forms usually an unpaired plate connected with 

 the median ends of the clavicles, and in those reptiles where 

 it occurs it is placed ventrally to the sternum proper. It is 

 expanded in front, and frequently takes the shape of a T, the 

 arms supporting the clavicles, while the shaft connects with or 

 may even be fused with the sternum proper. No episternum 



