VII.] 



SIRENIA. 



101 



In the Lesser Fin Whale (B. rostrata) the sternum is 

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

 the lateral arms of the cross. 



FIG. 41 Sternum of Rudolphi's Whale {Balcen&ptera borealis), showing its relation 

 to the inferior extremities of the first pair of ribs. From " Proceedings Zoological 

 Society, 1883." 



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

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

 segmentation. 



FIG. 42. Sternum of a young Dugong (Halicore indicus), \. From a specimen in 



the Leyden Museum. 



In a young Dugong (Halicore), Fig. 42, there are two 

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



