CHAPTEE XXX. 



CETACEANS, continued. 

 THE TOOTHED WHALES, OR ODONTOCETES. 



THE whole of the remainder of the existing Cetaceans form a group dis- 

 tinguished from the preceding one by the absence of whalebone, and the presence 

 of permanent teeth in at least the lower jaw. This group, comprising existing 

 families, is known as the Odontoceti, or Toothed Cetaceans, in contradistinction to 

 the Mystacoceti, or Whalebone Whales. 



In addition to the presence of teeth, the group is also characterised by the 

 following distinctive features. The tw r o nostrils unite before they reach the surface, 

 and thus open by a single aperture, which usually takes the form of a crescentic 

 valvular slit placed transversely to the long axis of the head. In the skull the 

 bones of the upper surface are arranged unsymmetrically on the sides ; the nasal 

 bones in existing forms being reduced to nodules, taking no share in roofing 

 over the cavity of the nostrils. The two branches of the lower jaw are nearly 

 straight, and of great vertical extent behind, while in front they come in contact 

 with one another by flattened surfaces of larger or smaller extent, as is ordinarily 

 the case among mammals. The ribs are more firmly joined to the rest of the 

 skeleton than in the whalebone whales, several of the anterior pairs articulating 

 with the bodies as well as with the horizontal transverse processes of the vertebrae, 

 while below they are joined to the breast-bone by the intervention of so-called 

 chest-ribs; the breast-bone itself usually consisting of several distinct portions, 

 placed one behind the other. In all cases the flippers have five digits. 



SPERM-WHALES AND THEIR ALLIES. 

 Family PHYSETERID^. 



The well-known gigantic sperm-whale is the typical representative of a family 

 characterised by the total absence of any functional teeth in the upper jaw; those 

 of the lower jaw being either numerous or reduced to a single pair. These characters 

 are sufficient to distinguish the members of this family from those of the two others 

 now existing, but it may be added that the skull is much elevated in the hinder 

 region, so as to form a high prominence or crest behind the aperture of the nostrils. 



The members of this family include the largest of the toothed Cetaceans, and 

 the whole of them are exclusively oceanic in their habits; their food consisting 

 mainly or solely of squids and cuttles. 



