vn.] UNGULATA. 79 



pair of ribs are close together on its upper surface ; but 

 the mesosternum is broad and flat, the first segment being 

 transitional, compressed in front, and broad posteriorly. The 

 xiphisternum is narrow and pointed. The sternum of the 

 Pig very often retains indications of the primordial median 

 fissure through life. 



The Horse and the Tapir have a very peculiar sternum. 

 The presternum is extremely compressed and projects forward 

 like the prow of a boat. In the Tapir, its anterior portion 

 is originally, and commonly remains, a distinct ossification 

 (pro-osteon, Parker). The segments which follow gradually 

 widen, and the hinder part of the sternum is broad and flat. 

 The last mesosternal segment in the Tapir is generally 

 divided in the middle line, and is not followed by a 

 xiphisternal element. 



The sternum of the Rhinoceros, on the other hand, is 

 very narrow throughout, with a long, rather spatulate xiphi- 

 sternum. 



Order Cetacea. — Each of the two primary divisions of 

 this order has a distinct form of sternum. 



Among the Odo7itoceti, the typical Dolphins have a very 

 broad presternum of peculiar form, emarginate in the 

 middle line in front, and with a pair of lateral processes 

 behind the attachment of the first pair of ribs. This is 

 followed by two or three mesosternal segments, but no 

 xiphisternum. An indication of the primordial median 

 fissure can generally be traced, except in very old animals, 

 either as a hole in the presternum, or as a division of the 

 posterior mesosternal segment. 



In the Porpoise (Phoccena) the sternum is shorter and 

 broader than in most Dolphins, and its various elements 

 early coalesce into a single bone. 



In the Cachalot (Physeter macrocephalus) the sternum 





