20 RESEARCHES UPON THE HYDROBIIN^ 



dina Nickliniana of Lea in the g;enus BythineUa. The other 

 American species, probably referable to the same genus, are 

 Amnicola attenuata, Haldeman, A. tenuijjes, Couper, A. obtusa 

 (Lea), Haldeman, and Pomatiopsis Binneyi, Trvon. 



The Bythinellae cannot be distinguished by the shell alone from 

 the brackish-water Littorinella3, and from several of the marine 

 Rissose. These two groups are as yet but little known, many of 

 their most important characters remaining' to be discovered. From 

 our present knowledge we can only state that the fresh-water genus 

 differs from'the RissoEe in the position of the basal denticles of the 

 rhachidian tooth of the lingual ribbon, and from the Littorinellae 

 in the obsolescc"nce of the armature of the outer lateral tooth, and 

 in the bifid verge. The verge in Littorinella miniUa (Ciugida 

 minuta, Gould), the only species I have examined, is simple and 

 not forked. Practically, of course, the difference in the element 

 they inhabit enaljles us to distinguish these groups, and may serve 

 the purpose until their characters are better understood. 



We now come to a group of American HydrobiinjB in which the 

 shell is globular in shape, and of large size, much larger, in fact, 

 than in any other Rissoids, and approached only, in this respect, 

 by the BythinifB and Potamopyrgi. The Amnicola isogona, Lea, 

 A. iniegra, Hald., A. aitilis (Lea), and A. Nuttalliaiia, Cooper, 

 may be mentioned as examples They were originally described 

 as Melaniae or Paludinas, but have, for the most part, been since 

 placed in Amnicola, although some authors have been inclined 

 to place them in the Melanian genus Leptoxis, to which they 

 have considerable resemblance, both in shell and operculum ; or 

 in Bylhinia, from which they differ much in both these hard parts. 

 The question of their true position is now solved by an examina- 

 tion of their soft parts ; they are undoubtedly HydrobiiniT!. I 

 have had opportunities of studying several species of the group, 

 and find among them three distinct genera, founded on the char- 

 acter of the verge and shell, which will be described below. One 

 of these, Somatogyrus, has already been indicated l^y Mr. Gill, 

 upon the characters of the shell in the Amnicola depi'ei<f<a of 

 Tryon. The genus Lithoglyphus of Mullfeldt, found in Europe 

 and South America, belongs to the same group. The "Amnicola 



