312 ORDERS OF MAMMALIA. 



the recently exterminated Ehytina occur in the Post-Pliocene 

 of Siberia. 



The genus Demothermm referred to this order by De 

 Blainville, and still retained in this position by Pictet, will be 

 here considered as belonging to the order of the Proboscidca. 



Order V. Cetacea. — In this order are the Whales, Dol- 

 phins, and Porpoises, all agreeing with the preceding in their 

 complete adaptation to an aquatic life. The lody is com- 

 pletely fish-like in form; the anterior limhs are converted into 

 sivimming -paddles or "fiippers ; " the pirooAmal hones of the 

 fore-limhs are much reduced in length, and the succeeding hones 

 are shortened and flattened, and are envcloiied in a tendinous 

 skin, tMis reducing the limbs to oar-like fins ; there are no 

 external ears ; the posterior limhs are completely absent ; and 

 there is a powerfid, horizontally -flattened, caudal fin, sometimes 

 accompanied by a dorsal fin as loell. In all these characters 

 the Cetacea agree with the Sirenia, except in the one last 

 mentioned. On tlie other hand, the nostrils, which may he 

 single or double, are always placed at the top of the head, con- 

 stituting the so-called " hloio-holes " or " spiracles ; " and they 

 are never situated at the end of a snout. The body is rery 

 sparingly furnished with hairs, or the adult may be completely 

 hairless. The head is generally of dispiropiortionately large size, 

 and is never separated from the body by any distinct constric- 

 tion or neck. The lumbar region of the spine is long, and, 

 as in the Sirenia, there is no sacrum, and the pelvis is repre- 

 sented by a single bone (the iscliium) on each side. A rudi- 

 mentary femur may be present, and Bcdcvna mysticetus has a 

 cartilaginous tibia as well. There are no clavicles, and some 

 of the digits may possess more than three phalanges each. 

 Lastly, the adult is cither destitute of teeth, or, with the single 

 exception of the Zcaiglodontidcc, is monophyodont — that is to 

 say, possesses but a single set of teeth, which are never re- 

 ])laced by others. When teeth are present, tltcy are usually 

 conical and numerous, and, except m tlie Zeuglodonts, they are 

 (diuays of one kind only. 



The Cetacea may be divided into the five families of 

 the Bcdamidm or Whalebone Whales, the Delpltinida^ or 

 Dolphins and Porpoises, the Catodontidm or Sperm Whales, 



