106 MOLLUSCA. 
The shell of the genus Fusus is lengthened, generally 
fusiform, destitute of longitudinal ribs, and bellied in the 
middle or lower part with a smooth pillar and lengthened 
canal. The /’.longicauda, (Lister, tab. 918, f. 11. A.) is the 
type. 
The Prevrotoma is distinguished from the preceding 
by a sinus or groove, which appears on the margin of the 
right edge of the mouth, near itssummit. It is represented 
by the M. Babylonicus of Linnzeus. 
The genus CLavatuta differs from the former, in pos- 
sessing a short canal, and ought never to have been separ- 
ated. 
In the genus CerrrHr1um, the mouth is oblique, termi- 
nating below in a short truncated or recurved canal, and 
having at the upper part a gutter more or less produced. 
The Tympanotonos asper of Mart. (Conch. 4. p. 314. Tab. 
156. f. 1473,) is the type of the genus. 
26. Trocuus. This is a very natural genus in the Lin- 
nzan system, and has undergone few alterations in the 
hands of modern conchologists. The T.perspectivus has 
given rise to a new and very obvious genus, termed Soia- 
RIUM, characterized by the internal spiral edge of the um- 
bilicus being crenulated. Another species, the T. dabzo, is 
the type of the genus MonoponTa, which contains shells 
of an oval form, with a rounded mouth, furnished with a 
tooth, formed by the truncated projecting base of the pil- 
lar: the two margins are separated. The turreted trochi 
of Linnzeus constitute the genus PyRAMIDELLA. 
The T. terrestris of British writers is so imperfectly de- 
scribed and figured, that it is impossible to assign it a place 
in the system. It is nearly allied to the helix. 
27. Turso. This very extensive genus has been greatly 
