TROCHUS. 



general form of the shell cannot be regarded as its most 

 important character, is evidenced by the fact that scarcely 

 two species have the same precise form, though for the 

 most part those which have a horny operculum with 

 numerous closely set volutions, such as Trochus Niloticus 

 and Turbo Pica, have a nearly similar general form, 

 although they are found to differ much in their peculiarities: 

 we have shown under Turbo that the Genus Monodonta 

 of Lamarck as well as Margarita of Leach must for the 

 most part be united to Trochus. We think, however, that 

 Rotella and Solarium have been properly separated from 

 Trochus. 



The Trochi are conical with a more or less elevated 

 spire, the axis of which being but slightly inclined and the 

 edge of the base of the cone being in general angular, 

 though it is sometimes rounded, they stand almost entirely 

 upon the base, which as Lamarck truly says is generally 

 flattish or even concave, as in Trochus Niloticus and 

 concavus, though sometimes convex, as in Trochus Pica, 

 (Turbo Pica SLUCtorum.) Their aperture is more or less 

 transversely depressed, (in the same degree as the base is 

 flat, concave or convex) in general rather quadrate or 

 trapeziform, and the dissimilarity in the form of the 

 aperture and the operculum is remarkable, the latter 

 being nearly circular. The edge of the aperture is oblique 

 to the direction of the last volution of the shell and shows 

 distinctly the inferior part of the columella, which is 

 constantly more or less arched ; in s<^me species the base 

 of the columella is truncated,* and forms a more or less 

 distinct tooth, (some of these form part of Lamarck's 

 Monodonta;) in others it is continuous with the peritreme 

 and has not the least vestige of a tubercular tooth ; there 

 is, however, a regular gradation from the one to the other. 

 Operculum horny, circular, spiral, with numerous, close- 

 set volutions, and having the spiral line externaU 



The Trochi are marine^ and they are found almost 

 everywhere, many are common on our own coasts, several 

 of which are very beautiful, such as the Tr. Magus, Tr. 

 cinerarius, Tr. ziziphinus and others. Some of the larger 



* It is observable that although Lamarck gives this as the principal 

 distinguishing character of Monodonta, the same character is to be remarked 

 in many of his Trochi. 



