44 A BOOK OF WHALES 



much as they bear upon the broad lines of modifica- 

 tion which these aquatic mammals have undergone 

 in their gradual change and adaptation to a life in 

 the ocean. 



The typical mammalian breast-bone consists of a 

 number of separate pieces of bone, often spoken of as 

 " sternebrae," and forming a row along the middle line 

 of the breast. Between each of these separate bones 

 is inserted a rib. The number of pieces out of which 

 the sternum is formed is sometimes very large ; as 

 many as fourteen elements occur in the Sloth (Cholcepus) 

 for instance. Among the toothed whales the sternum 

 shows what we must regard from a comparison with 

 land mammals as the most primitive conditions. In 

 Berardius, for example, the sternum consists of five 

 pieces placed end to end, and these bear facets for 

 six ribs. A very interesting feature of this sternum 

 is to be seen in the fact thfat it is not only distinctly 

 bifid behind, but that it is also somewhat incomplete 

 in the middle line, gaps being left in the dried 

 skeleton from which probably pieces of cartilage have 

 dropped out. Now the interest of what seems to be 

 a mere detail of anatomy is this : the sternum of 

 mammals is developed from a fusion between the 

 lower ends of the growing ribs ; it is at first in two 

 longitudinal pieces, and the ossification -- the con- 

 version into bone of this cartilage is also double, 

 paired centres of the deposition of bony matter 

 appearing. Thus in Berardius (and in other forms) 

 distinct traces of the original paired state of affairs 

 are left. In other toothed whales the number of 



