GASTEROPODA. 3 



Shell — Almost triangular; regular; inequilateral ; liinge de- 

 void of teeth ; ligament linear, internal. It includes 

 only one _^e^zM5— Dreissena. 



The U7ii'calves come next under notice. 



CLASS II.— GASTEROPODA. 

 Section L — Gasteropoda Prosobranchiata. 



So called from the animal moving on its stomach and 

 having its organs of breathing in advance of the head. 

 The mouth is usually at the end of a short proboscis, 

 with two tentacles ; eyes either placed at the hinder 

 edge of the tentacles, on a separate pedicel, or sessile, at 

 their base ; branchia pectinated, or plume-like, in the 

 form of one or more extended filaments, through which 

 they respire the air of the water ; the mantle in which 

 the excretory orifices are placed, is entire, and without 

 syphon, forming a receptacle for the breathing organs 

 in the shape of a vaulted chamber over the head ; ab- 

 domen well developed : sexes distinct. The shell, pro- 

 tecting partly the gills, and partly the other parts of the 

 body, is spiral, with the mouth either entirely round, 

 or half-moon shaped, covered when the animal has re- 

 tired within its shell, with a horny operculum variously 

 shaped. 



In this section there are three tribes found in our 

 fresh waters. 



Tribe I. — Neritid^e. 



Animal — With moutJi semiovate ; tentaculcB slender, awl- 

 shaped ; eyes on prominent pedicels, at the hinder edge 

 of the tentacles ; foot rather short, oblong, triangular ; 

 tongue complicated in its structure, and denticulated ; 

 sides of the body simple. 



Shell — Thick, semi-globose, imperforate ; spire with a few 

 whorls only, last whorl very large and expanded ; outer 

 Up acute ; columella fattened ; surface covered with an 

 epidermis ; the operculum, which is spiral and has a tooth 

 at the lower edge^ just fits the semi-ovate mouth of the 

 shell to which it is attached by two internal processes, 



B 2 



