66 LAND AND FRESH-WATER SHELLS OF N. A. [PART IIL 



by Troschel) on each side, wliicli may be either on the basal 

 margin, or on the anterior surface of the tooth above the 

 base ; cusp recurved and denticulated. Intermediate tooth 



Lingual dentition of Amnicola fayana. 



more or less hatchet-shaped, having a handle-like process 

 (peduncle) projecting outwardly from the base of the broad 

 body which is denticulated at the upper margin. Lateral 

 teeth generally slender and armed with numerous minute 

 denticles at their superior margins. Shell small, spiral, tur- 

 reted or depressed, often more or less umbilicated ; aperture 

 more or less rounded, never truly channelled in front ; peri- 

 treme continuous. Tentacles elongated, with the eyes at their 

 outer bases. Verge (male organ) exserted, situated on the 

 back at a considerable distance behind the right tentacle. 

 Gills both pallial; the right or principal one usually rather 

 short and broad, and composed of few lamina?, which are 

 much broader than high. Foot oblong, truncate before, 

 rounded or pointed behind. Operculigerous lobe well de- 

 veloped. Operculum horny or partly shelly, spiral or con- 

 centric. 



Station in fresh, brackish, or sea water, rarely on land. 

 Distribution mundane, — \_Stimpson^ 



Dr. Stimpson subdivides the Bissoidae into the following sub- 

 families : — 



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 Rissoina, D'Orb. (See 

 Stimpson 's paper, p. 39.) 



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

 operculum not provided with a process. Foot without lateral 



