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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 ; 
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. 
