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TROCHUS. 
A. Erect, pillar perforated. (Plate XIX. Fig. 1.) 
B. Imperforate, erect; the umbilicus closed. 
(Fig. 2.) 
C. Tapering, with an exserted pillar, and fall- 
ing on the side when placed upon the 
base. (Fig. 3.) 
Shell univalve, spiral, subconical. Aperture four- 
sided, and somewhat angular ; or more round, 
having the upper part of the margin converging 
towards the pillar. Columella oblique. 
Some species, the aperture of which tends to a 
circular or oval form, are distinguished by a tooth- 
like projection. There are no two genera more 
confounded with each other, or more difficult to be 
discriminated by the inexperienced student, than 
this and the following one. It is scarcely possible 
to define the boundary at which the Trochi with 
rounded apertures are supposed to end, and the 
Turbines with imperfectly circular mouths to begin 
their jurisdiction. The true form of the Trochus 
