102 



NERITA. 



A. Umbilicate. (Plate XX. Fig. 6.) 



B. Imperforate, with the lips toothless. (Fig. 7.) 



C. Imperforate, with the lips toothed. (Fig. 8.) 



Shell univalve, spiral, gibbous, rather flat under- 

 neath. Aperture semiorbicular, or semilunar 5 

 having, uniformly, the pillar lip, or columella 

 straight. 



In no one genus is the diagnosis more perfect 

 than in Nerita. The third division is totally un- 

 like all others : the flat, toothed, inner lip and 

 narrow throat, which constitute its very obvious 

 characters, are not to be found in the slightest de- 

 gree of similarity in any but this one family. A 

 few species of the section B. resemble Turbines; 

 but their columella is always to be distinguished by 

 its flatness. The umbilicate shells might in some 

 instances be misplaced in the genus Helix, the 

 whorls having much the same external figure and 

 simplicity of colour ; the straight pillar lip is, how- 

 ever, in all these individuals sufficiently evident, 



