10 



RESEARCHES UPON THE HYDROBIIN^ 



6oft parts of his genus Bi/ihinia' (in ■uiiicli he includes not only 

 the true Bythinite but the Bythinellae also^), except in leaving out 

 the expression "a tortillon spiral," in relation to the entire ani- 

 mal, and in the substitution of the more nearly exact term " Jaws 

 obsolete" for " Machoires nulles." The characters are used for 

 the group originally founded upon our American Amnicola, on 

 the assumption that our American species agreed therein with the 

 European forms studied by Moquin-Tandon. The diagnosis will 

 not, however, apply to our American forms as a group. The foot 

 is by no means " narrow" in the greater part of our species. The 

 jaws are not "obsolete;" — I have found them present and suffi- 

 ciently well-developed in Amnicola porala and all others which 

 have come under my 'observation.' The tentacles are not "seta- 

 ceous, pointed," in Amnicola proper, but conspicuously of equal 

 size throughout their length, and truncated at their extremity. 

 Finally, the verge is not bifid in all of our species. 



Having eliminated these false characters, we can more easily 

 determine whether these Amnicol^e, and their allies, are entitled 

 to rank as a family distinct from the Rissoida), in which the 

 typical forms were placed by H. & A. Adams. AVe find, however, 

 no character left which will serve to distinguish them, with the 

 exception of " foot not continued in front of the rostiaim." But 

 this character does not seem to be of sufficient importance to 

 indicate the separation of the two groups as distinct families, 

 when the agreement is so close in all other points. It is also a 

 very uncertain character. In describing these animals, sufficient 

 care has not been taken to mention their position or movement 

 at the time the description is drawn up. Among the figures 

 of Rissofe in the great work of Forbes and Ilanley on the 

 Bi'itish Mollusca, we find some species represented with the head 

 in advance of the foot, and others with the anterior extremity 

 of the foot in advance of the head. On the other hand, I have 



' MoUusques terrest. et fluv. de France, II, 514. 



^ Moquin-Tandou's rather unnatural approximation of these two groups 

 seems to have been chiefly founded on the similarity of their generative 

 organs, which are strikingly different from those of Viviparus, to which 

 genus the Bythinise were formerly referred. 



' That they exist also in Bi/fhinia, notwithstanding the statement to the 

 contrary by Moquin-Tandon, has been discovered by Troschel (see "Gebiss 

 der Sflmecken," I, 162). Moquin-Tandon himself admits having found 

 traces of them in Bj/thinella viridis (op. cit., II, 525). 



