1. 1 



CETACEA. 



8l 



resulting pieces are not united by bone across the middle 

 line, while the second or last pair of mesosternal segments 

 (ms 2 ) are ankylosed together mesially, but not with the 

 portion of the sternum in front of them. 



In the Whalebone Whales (Mystacoceti) the sternum is 

 comparatively rudimentary, consisting only of a broad, 

 flattened presternum, produced posteriorly into a xiphoid 

 process in some species. There are never any mesosternal 

 segments, and consequently no ribs, other than the first 

 pair, are attached to it. 



). — Sternum of Common Rorqual or Fi 

 Whale {Balcenoptera musculus), fa. 



Fig. 40. — Sternum of Pike Whale 

 {Balcenoptera rostrata), J . 



The presternum is ossified from one, or perhaps a pair of 

 symmetrical nuclei. In the Right Whales (Balcena, Fig. 38) 

 it is heart-shaped, or longitudinally oval. In the Fin Whales 

 [Balcenoptera) it is transversely oval or trilobate, with a 

 backward projecting xiphoid process. 1 In young animals 



1 In the cartilaginous sternum of a young Balanoptera sibbaldii 

 Professor Turner found the xiphisternum to be quite distinct from the 

 presternum, and connected with it by fibrous tissue. {Journal of 

 Anatomy, May 1870.) In most Whales the sternum shows no such 

 evidence of segmentation. 





