98 THE STERNUM. [CHAP. 



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-ostcon, 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. 



The sternum of the Rhinoceros is very narrow through- 

 out, with a long, rather spatulate xiphisternum. 



Order CKTACF.A. Each of the two primary divisions of 

 this order has a distinct form of sternum. 



Among the Odontcceii, 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 (P/iocfcna) 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 

 ossifies from three distinct pairs of nuclei, and a large 

 median fontanelle remains between the first and second 

 pair. 1 In the specimen in the Museum of the Royal College 

 of Surgeons (Fig. 37), which is very nearly adult, each half 

 of the presternum (ps) has coalesced with the corresponding 

 half of the first segment of the mesosternum (;//-$'), but the 

 resulting pieces are not united by bone across the middle 

 line, while the second or last pair of mesosternal segments 

 1 I have observed this in animals evidently of great age. 



