FAUNA AMERICANA. 141 



Species. 



5. Arvicola floridanus> 



Mus Jloridanus, Ord, Nouv. bull, de la Soc. Phi- 



tion than a slight and unimportant variation in the form and 

 direction of the plates of enamel, which traverse the crowns of 

 the molars, and the partial division of the root into rudimentary 

 radicles. On similar distinctions, F. Cuvier has founded his divi- 

 sions of the genus Arvicola^ which " differ from each other in the 

 number of parts of which the teeth are composed." 



It could be shown, if necessary, in a number of instances, that 

 greater differences are observable in the different teeth of the 

 same individual, than have served those gentlemen, in the pre- 

 sent instance, to construct a new genus; who, in their descrip- 

 tion of the animal, have entirely neglected to point out any 

 generic distinctions in the external characters^ (which, in reality, 

 correspond with the genus Arvicola.) This neglect is the more 

 extraordinar}', as in their former descriptions they have dwelt 

 upon the external characters of animals, and, in some cases, to 

 the exclusion of any observations on the structure of the teeth, 

 as was instanced in the " Mus Jloridanusy The slight variations 

 in the teeth noticed above, provided they be accompanied with 

 well marked differences in the external characters of the ani- 

 mal, may form good grounds for specific distinctions, but surely 

 cannot be received as sufficient reason for the construction of a 

 new genus, according to the established laws which regulate 

 naturalists in similar instances ; particularly as nature acknow- 

 ledges no such distinction, inasmuch as the food, the manners, 

 the habits, and we may add, the external characters of this ani- 

 mal, correspond with those of other species of the genus Arvicola. 



We have dwelt the longer on the principles involved in this 

 dispute, believing that, if the precedent be established, it might 

 prove fatal to the best interests of science. 



Note. — The description of the present species is drawn from 

 one of the three individuals presented to the Museum by Mr. Ord. 



