OF CONCHOLOGY. 153 



except that of "foot not continued in front of rostrum. 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." For this 

 reason, our author forms a subfamily, Hydrohiinve^ for these 

 shells. The Rissoidne are thus described : — 



"Tentacles elongated, with the eyes at their outer bases. 

 Verge (male organ) exserted, situated on the back at a con- 

 siderable distance behind the right tentacle. Gills both pal- 

 lial; the right or principal one usually rather short and 

 broad, and composed of few laminae, which are much broader 

 than high. Foot oblong, truncate before, rounded or pointed 

 behind. Operculigerous lobe well developed. Operculum 

 horny or partly shelly, spiral or concentric. Lingual teeth 3. 

 1. 3 ; the rows being more transverse and less arcuated than in 

 the Littorinidoe. Khachidian tooth broader than long, and 

 armed with basal denticles (so called by Troschel) on each 

 side, which may be either on the basal margin or on the 

 anterior surface of the tooth above the base ; cusp recurved 

 and denticulated. Intermediate tooth more or less hatchet- 

 shaped, having a handle-like process (peduncle) projecting 

 outwardly from the base of the broad body, which is denticu- 

 lated at the upper margin. Lateral teeth generally slender 

 and armed with numerous minute denticles at their superior 

 margins. Shell small, spiral, turreted or depressed, often 

 more or less umbilicated ; aperture more or less rounded, 

 never truly channelled in front ; peritreme continuous. Sta- 

 tion in fresh, brackish, or sea water, rarely on land. Distribu- 

 tion mundane." 



In this family are included, according to Dr. Stimpson, 

 several subfamilies, which he names and describes as follows : 



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

 which is calcareous within, and with cervical lobes. They 

 are comparatively large. Fresh water. Genus Bythinia* 

 Gray. 



EissoiNiN"^, with an ovate or turreted shell, and a thick, 

 corneous, subspiral operculum provided with an internal pro- 

 cess (articulated). Size small. Marine. Genus Rissoina, 

 D'Orb. 



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

 operculum not provided with a process. Foot without lateral 

 sinuses. Khachidian tooth of the lingual ribbon with the 

 basal teeth on the inferior margin. Size small. Marine. 

 Genera Rissoa, Frem., Gingula, Flem., Alvania, Risso, Onoba, 

 H. and A. Ad., Setia, H. and A. Ad., Ceratia, H. and A. Ad.^ 

 Fenella, A. Ad. 



* Syn. Elona^ Moq.-Tand. 



