found to want essential distinctions sufficient to constitute 

 genera. Confining our attention to such of them as 

 have representatives in this country, we find that Arion 

 is not universally received, and that its diagnostic char- 

 acters are by some thought to be of little importance. 

 We shall not enter into this question, but shall only 

 remark, that unless the whole family is reduced to a 

 single genus, as it stood in the time of Linnaeus, it will, 

 in consequence of the general similarity of their organi- 

 zation, be necessary to found the genera upon slight 

 differences of structure. Considerable difference exists 

 between the structure of Limax and Arion; besides 

 which, as the former has become numerous in species, it 

 is desirable, as a matter of convenience, to subdivide it. 

 We have therefore adopted the latter genus. 



Very little attention has hitherto been given, in the 

 United States, to the animals of this family. The only 

 paper on the subject which we have met with, excepting 

 that published by ourselves, in the Journal of the Bos- 

 ton Society of Natural History, is one by the late M. 

 Rafinesque, contained in one of his abortive periodicals 

 entitled "Annals of Nature" for 1820. In this paper 

 that author, with his usual facility in proposing new 

 genera and species, gives the characters of two genera 

 and six species, from animals noticed by him in various 

 parts of the country, but not since recognized by other 

 naturalists. We have discussed the character of M. 

 Rafinesque in the preface, and have given the reasons 

 why we cannot adopt his proposed genera ; we have also 



