CETACEA. 



8i 



resulting pieces are not united by bone across the middle 

 line, while the second or last pair of mesosternal segments 

 (;///-) are ankylosed together mesially, but not with the 

 portion of the sternum in front of them. 



In the Whalebone Whales {Mystacoceti) tlie 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. 



FiG. 39.— Sternum of Common Rorqual or Fi 

 Whale {BalcefioJ>tera vmscidns), i'^. 



Fh;. 40. — Sternum of Pike Whale 

 {Balcpnoptera rosfrata), yg. 



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

 symmetrical nuclei. In the Right Whales {BalcE/ia, Fig. 38) 

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

 {Bal(E7ioptera) it is transversely oval or trilobate, with a 

 backward projecting xiphoid process.^ In young animals 



^ In the cartilaginous sternum of a young BaLcnoptcra sibbaldii 

 Professor Turner found the xiphistenium 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 sucli 

 evidence of segmentation. 



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