190 

 PLANORBIS. 



/-£\ \ HELIX CORNU-ARIETIS, LinntBUS. 



Encycl Melh. plate 460, fig. 3, a, b. 



Chem. 9, 1. 1 12, / 952, 953. 



This genus formed a part of the G. Helix of Linnaeus. 

 Muller and Bruguiere were the first to separate it; and 

 Lamarck agrees with them in the propriety of so doing, 

 not only with a view to diminish the great number of 

 shells described as belonging to the G. Helix, but to dis- 

 tinguish the aquatic from the terrestrial species. Shells 

 of this genus are discoid, that is, having the spiral evo- 

 lutions on an horizontal plane, or, as it were, wound 

 round a central point, gradually increasing in size, and 

 leaving the upper and lower sides concave, and nearly 

 similar, the spire being only a little elevated; the aper- 

 ture oval, dilated, far removed from the axis of the shell ; 

 the lip never reflected; substance generally thin and 

 diaphanous; the whorls nearly cylindrical, but some- 

 times flattened, angular, orcarinated; they have no 

 operculum, and inhabit fresh water. 



Lamarck does not appear to have observed, in more 

 tlian one species of the P.spirorbis, the very peculiar 



