AND ALLIED FORMS. 39 



from Bythinella and Paludestrina, and the same difficulty will 

 probably be found with Gillia and Somatogijrus, or with Litho- 

 ghjphus and Fluminicola. I shall therefore under each genus give 

 as examples only a few species which certainly belong to it, without 

 attempting to assign a place to every known species of the family. 

 It would not, of course, be difficult to do this approximately, but 

 rather than run the risk of adding to an already overburdened 

 synonymy, I will leave the work to those who have proper oppor- 

 tunities for observing the entire animal in each case ; and would 

 beg my fellow-workers in this field ta take the same course. 



Subfamily HYDROBIIN^. 



Shell very small, or of moderate size, never exceeding two- 

 fifths of an inch in length, globose, ovate, or elongated, generallv 

 umbilicated or rimate, and covered with a periostraca for the most 

 part of an olive color; whorls moderately numerous (4-8), smooth, 

 or, rarely, ribbed or carinated, never cancellated ; aperture more 

 or less ovate or rounded, rarely subacute or effuse anteriorly ; 

 peritreme continuous ; outer lip usually simple and acute. Oper- 

 culum subspiral, corneous or testaceous. Tentacles, verge, and 

 gills as in the diagnosis of the family, p. 3. Foot without lateral 

 sinuses, truncate and auricled in front, and generally rounded 

 behind; operculigerous lobe destitute of cirri. On the lingual 

 ribbon the rhaciiidian tooth is much broader at the base than at 

 the summit, with the basal margin trilobate, and the basal deo- 

 ticles situated on the anterior surface, between the base and the 

 oblique lateral margins, being connected with these margins by a 

 carina or lobe usually extending to the infero-extcrior angle of 

 the tooth ;, so that they are rather dependencies of the lateral 

 margins than of the base. The peduncle of the intermediate 

 tooth is slender and generally long. The lateral teeth are straight 

 or regularly curved, with no approach to the sigmoid form- seen 

 in the Rissoinae and Skeneinse. 



Station, in fresh or brackish water. 



Like all of the Rissoidae these little animals are strictly herbivo- 

 rous. Moquin-Tandon remarks* that they have, connected with 

 the stomach, a cartilaginous stylet like that occurring in certain 



' Hist. Nat. des Moll, ter. et fluv, de France, II, 514. 



