174 On the Classification of Mammalia. 



are the smallest of the order, and possess two fresh-water 

 representatives, one closely allied to dolphins proper, the 

 second bearing some far relations to Physeteridse (sperm- 

 whale), and to the genus Hyperoodon of the heterodonts 

 family. 



The morphology of the teeth in Cetacea is very interest- 

 ing, and instructive in a philosophic point of view, when the 

 relationships of this order with the Edentata are well under- 

 stood. In the lowest type, teeth remain undeveloped ; in 

 the highest, they cover the whole surface of both jaws, but 

 are of one kind : incisors, canines, and grinding teeth are not 

 known amongst cetaceans. This fact alone would ascribe to 

 them an inferior rank amongst the normal groups of the class. 



§ 4. The affinities of the so-called herbivorous cetaceans, or 

 Sirenidse, with pachyderms, have been alluded to by several 

 authors. In 1834 Fred. Cuvier* wrote the following re- 

 markable sentence : " The group of herbivorous cetaceans, 

 composed of genera intimately connected together, are related 

 to the pachyderms by the manati." And farther on (page 6) 

 he remarks that they come nearer to pachyderms than to 

 cetaceans. In 1838 they were definitively removed from the 

 Cetacea, and actually placed amongst Pachydermata.t Upon 

 this point, every naturalist now agrees. Sirenidse are the low- 

 est grade among pachyderms ; even if considered as parallel 

 to pachyderms, they still must rank lower in a natural classi- 

 fication. They are aquatic, provided only with the anterior 

 limbs constructed for swimming. Unlike the cetacea, they 

 live near the land, and may occasionally creep along a beach ; 

 undoubtedly representing a higher step in the class, and an 

 approximation towards the subaquatic Hippopotamus, which, 

 together with the tapir, shew intimate relation with the 

 manati and dugong. The Dinotherium, and other fossil re- 

 presentatives of the group of Sirenidia, seem to synthetise 

 the living genera of their groups, together with both the 

 proboscidian pachyderms and the ruminants. This synthesis, 

 however, cannot yet be fully understood. The earth's crust 



* Ilistoire Naturellc des Oetaces, p. 34. 

 t Owen, in Proceed. Zool. Soc, London. 



