AND ALLIED FORMS. 3 



Tentados elongated, with the eyes at their outer bases. Vcvg-e 

 (male organ) exserted, situated on the back at a considerable 

 distance behind the right tentacle. Gills both pallial ; tlie 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 con- 

 centric. Lingual teeth 3. 1. 3 ; the rows being more transverse 

 and less arcuated than in the Littorinidae. Rhachidian 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) project- 

 ing 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 um- 

 bilicated ; aperture more or less rounded, never truly channelled 

 in front; peritreme continuous. Station in fresh, brackish, or 

 sea water, rarely on land. Distribution mundane. 



The family Rissoidae, as now circumscribed, notwithstanding 

 their agreement among themselves in all characters which are of 

 importance for the discrimination of the family, yet present such 

 considerable differences in minor details, that they are found to 

 arrange themselves naturally into several distinct groups, or sub- 

 families. We find genera in which the shell is turreted and 

 elongated, and others in which it is globular or depressed ; some 

 in which the verge is bifid, and others in which it is simple ; some 

 which have long proboscidiform snouts, and others with extremely 

 short ones ; some with lateral sinuses in the foot, and others with- 

 out them ; some with the foot produced anteriorly, and others 

 having it shorter than the snout ; some with a cirriform appendage 

 to the operculigerous lobe, and others without ; some with a spiral, 

 others with a concentric operculum ; and these differences are in 

 some cases coincident with the great diversity in station and 

 habits which we observe among these little snails. As already 

 noticed, they inhabit the greatest possible variety of station, some 

 of the genera being marine, and living even at great depths in the 



