TRO CH U S, 109 



their jurisdiction. The true form of the Trochus 

 is that of a pointed cone, capable of standing 

 nearly perpendicularly, or but little inclined, upon 

 the flattened base of the last whorl : the aperture, 

 broader than long, angular at the lower extremi- 

 ties of the columella, and at the carinate margin 

 of the outer lip, is so situated as to be nearly hori- 

 zontal when the shell is placed in an upright pos- 

 ture. From this most perfect structure, to the ver- 

 tical and circular formation, there are so many 

 gradations and varieties of aperture, that ii were 

 endless to describe them. The most simple, 

 though not an unexceptionable rule is, to consider 

 all specimens as belonging to this genus which 

 have any angular tendency in the contour of the 

 mouth, and are, as to their general appearance, 

 /op-shaped, in conformity with the meaning of 

 their Latin name. 



