66 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



In the birds (fig. 60) the sternum is ossified and at most is con- 

 nected with eight pairs of ribs. Behind, it may be rounded, per- 

 forated, notched, or prolonged into one or two long processes. In 

 the ostriches the ventral surface is smooth and this was formerly 

 used as a character separating these birds as a group of ratitae, in 

 contrast to all other birds (carinatae) which either use their wings 

 in flight or in swimming (penguins) and in which there is a necessity 

 for strong wing muscles. For the attachment of these the ventral 

 surface of the sternum is developed into a strong projecting keel 

 (carina). It is to be noted that a similar keel is developed in the 

 bats and pterodactyls. 



In the mammals the number of ribs connected with the sternum 

 is greater than in the lower classes. The sternebrae may remain 

 distinct throughout life (fig. 65) or, as in man, they may fuse into 

 fewer elements, the xiphoid process being unconnected with the ribs. 

 In the edentates and rodents elements known as ossa suprasternalia 

 and prosternum occasionally occur in front of the sternum, the sig- 

 nificance of which is unknown. It is possible, that traces of them 

 persist in the higher orders, even in man (fig. 62, ss). 



Episternum (Interclavicle) 



In stegocephals and the oldest rhynchocephals there is a median 

 bone on the ventral surface, called the episternum (fig. 66). It is 



Fig. 66. — Shoulder girdles of (^4) Melanerpeton and (B) diagram of Branchiosaurus, 

 after Credner, the determination of elements after Woodward, cl, cla\acle; co, cora- 

 coid; e, episternum; s, scapula. 



rhomboid in front and may have a long posterior process, the medial 

 ends of the clavicles lying ventral to the broad anterior end. This 

 element is regarded as homologous with a T-shaped membrane bone 

 which occupies a similar position in lizards (fig. 64) and crocodilians, 

 where it acts as a brace between the shoulders. It arises in mem- 

 brane by two centres of ossification and hence cannot be the same 

 as the suprasternalia of mammals. An episternum also occurs in 



