THE SIRENIA, OR SEA-COWS. ' 243 



eastern shores of America. Another and fourth 

 species of very remarkable form, the Rhytina 

 stelleri, belonged to our present period, but owing 

 to the smallness of the range of its distribution, 

 seems to have become extinct between the years 

 1741-48. 



The earlier systems of zoology considered the 

 want of hind legs in Whales and the Sirenia,, the 

 paddle- shaped form of their front limbs, and! the 

 formation of the end of the body into an horizon- 

 tally extended fin, to constitute the characteristic 

 features of a distinct order of animals, compared 

 with which other very marked differences in their 

 skull and dentition seemed of little importance. 

 However, nowadays we are so well acquainted 

 with the disappearance of the front or back 

 limbs, or of both extremities (in the case of 

 reptiles) as phenomena of convergence, without 

 this being considered a proof of any near blood- 

 relationship, that we no longer think of classing 

 the Sirenia with the Whales simply because of the 

 want of the hind limbs. The Whales are flesh- 

 eaters, the Sirenia plant-eaters ; the former, by 

 their relationship to the seals, belong to the order 

 of the Fera, flesh-eaters in the narrower sense of 



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