82 THE STERNUM. [chap. 



ossification of the cartilaginous sternum advances forwards 

 on each side of the middle line, so that the ossified portion 

 at one period appears deeply notched in front ; as the bone 

 meets across the middle line anteriorly, this notch usually 

 becomes converted into a hole (see Fig. 39), which finally 

 closes with complete maturity. 



In the Pike Whale (B. rostraia) the sternum is cross 

 shaped (Fig. 40), the first ribs being attached behind the 

 lateral arms of the cross. 



In the Sirenia the sternum is a simple, flattened, some- 

 what elongated bone, which in the adult shows no trace of 

 segmentation. 



iy 



* 



!: 



Fig. 41. — Sternum of a young Dugong (Halicore indicus), \. From a specimen 

 the Leyden Museum. 



In a young Dugong (Halicore), Fig. 41, there are two 

 distinct ossifications, — a presternum (ps), to which the first 

 pair of ribs are attached, and a xiphisternum (xs). The 

 second, third, and fourth pairs of sternal ribs are attached 

 to the intermediate unossified portion, representing a rudi- 

 mentary mesosternum. 



