82 THE STERNUAI. [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 AV'hale {B. rostmta) 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. 



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

 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 raesosternum. 



