402 ZOOLOGY. 



d. TrichechidcB. 



e. Sirenidia. 



Rytinidae, Halichoridae, Manatidae, Dinotheridas. 

 VI. Cetacea. 

 , Heterodontidae, Delphinidse, Physeteridae, BalsenidaB. 



VII. Marsupialia. 

 a. Carnivora. 



Thylacinidae, Didelphidse, Dasyuridae. 

 h. Insectivora. 



Peramelidse. 

 c. Herhivora. 



Phalangistidae, Phascoloipiyid8e,Macropodidae (Halmaturidee). 



VIII. Edentata. 

 a. Tardigi'ada. 



Bradipodidae, Megatheridae. 

 h. Edentata proper. 



Manidae, Myrmecophagidae, Orycteropodidae, Dasypodidae. 

 c. Monotremata. 



Echidnidae, Ornithorhynchidae. 



A glance at the actual and terrestrial mammalian fauna of North 

 America shows a scarcity of the types which we have placed at the bottom 

 of the class : the two great groups of Edentata and Marsupiaj^ia have 

 each but one species, both confined to the warmer part of the continent. 

 The Pachydermata are represented by a single species also, not taking into 

 consideration the introduced species, the horse, the ass, and hog. The 

 Ruminantia are distributed into eight genera, seven of which comprise but 

 one species, the introduced not included. The Rodentia are the most 

 numerous : they form twenty-one or twenty-two genera, and count from 

 eighty to ninety species ; the rodents, it must be observed, are among the 

 smallest mammals, and therefore strike the attention less than either the 

 ruminants or pachyderms. Insectivora, twenty to twenty-five in number, 

 are arranged into six genera. The Carnivora, the number of which is a 

 little above thirty, are distributed into fourteen genera, several of which 

 have only one species. The viverrine carnivora are almost completely 

 absent. If future investigations shall increase the number of North Ame- 

 rican mammals, it cannot be but in favor of the rodents mostly : that 

 group, therefore, may be considered as the most numerous in North Ame- 

 rica ; next the carnivora proper ; next the insectivora, then ruminants. 

 The reasons for such proportions must be sought for in the physical condi- 

 tion of the continent : the small number of grass feeders is a very remark- 

 able fact, and doubtless in direct relation with the proportion of carnivora. 

 Thus America, although the continent of vegetation, has not received a 

 proportional number of herbivora, showing that the equilibrium between the 

 different orders of animals is more important than that of the vegetable and 

 animal kingdom. 

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