4 RESEARCHES UPON THE HYDROBIIN^ 



ocean, while others live in brackish water, many in fresh water, 

 and one, at least, habitually on land. 



Upon these grounds I would suggest the division of the family 

 into the following subfamilies, using the characters of the soft 

 parts, as well as those of the shell and operculum : — 



Bythiniin^, with an ovate shell, a concentric operculum which 

 is calcareous within, and with cervical lobes. They are compara- 

 tively large. Fresh water. Genus Bythinia,^ Gray. 



RissoiNiN^, with an ovate or turreted shell, and a thick, corne- 

 ous, subspiral operculum provided with an internal process (articu- 

 lated). Size small. Marine. Genus Bissoina, D'Orb. 



RissoiN^, with an ovate or elongated shell, and a subspiral 

 operculum not provided with a process. Foot without lateral 

 sinuses. Rhachidian tooth of the lingual ribbon with the basal 

 teeth on the inferior margin. Size small. Marine. Genera Rissoa, 

 Frem., Gingida, Flem., Alvania, Risso, Onoba, H. & A. Ad., 

 Setia, H. & A. Ad., Ceratia, H. &'A. Ad.,Fenella, A. Ad. 



Skenein^e, with a depressed, almost discoidal shell, and a 

 corneous, paucispiral" operculum. Minute. Marine. Genus 

 Shenea, Flem. 



Hydrobiin^, with shell and operculum and foot like those of 

 the Rissoin^, but with the rhachidian tooth of the lingual ribbon 

 having the basal teeth on the anterior surface, behind the lateral 

 margins. Size variable ; some are minute, some as large as 

 Bythinite. Living in fresh or brackish water. Genera Hydrobia,^ 

 Hartm., Littorinella, Braun, Amnicola, Gould «&; Hald., Bythi- 

 nella* Moq.-Tand., Slenothyra,^ Benson, Tricula, Benson, Pyr- 

 gula, Christ. & Jan., Paludestrina, D'Orb., Tryonia, Stm., 

 Potamopyrgus, Stm., Lithoglyphus, Muhlfeldt, Fluminicola, 

 Stm., Gillia, Stm., Somatogyrus, Gill, Cochliopa, Stm. 



PoMATiopsiNiE, with the shell and operculum as in the Ris- 

 soinse. Foot with lateral sinuses. Size small. Amphibious. 

 Genus Pomatiopsis,^ Tryon. 



' Syn. Elona, Moq.-Tand. 



2 According to the terminology of Woodward, the opercuUim of Skenea 

 would be multispiral, and that of the Rissoinse, etc. paucispiral. 



' Syn. Paludiiiella, Loven (not Pfeiffer), and Littorinida, Eyd. & Soul. 

 * Syn. Leachia, Risso (not Lesueur), Microtia, Ziegler. 

 ' Syn. Ncinatura, Benson. 

 ^ Syu. Chilocydus, Gill. 



