GASTEROPODA. O 



Shell Almost triangular; regular; inequilateral ; liinge de- 

 void of teeth ; ligament linear, internal. It includes 

 only one genus Dreissena. 



The Univalves come next under notice. 



CLASS II. GASTEROPODA. 

 SECTION I. 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 mouth semiovate ; tentaculce 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 

 lip acute ; columella flattened ; surface covered with an 

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

 at the lower edge, just Jits the semi-ovate mouth of the 

 shell to which it is attached by two internal processes, 

 B 2 



