96 



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, subcorneal. 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 



