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 F. longicauda, (Lister, tab. 91 8, f. 1 1 . A.) is the 

 type. 



The PLEUROTOMA is distinguished from the preceding 

 by a sinus or groove, which appears on the margin of the 

 right edge of the mouth, near its summit. It is represented 

 by the M. Babylonicus of Linnaeus. 



The genus CLAVATULA differs from the former, in pos- 

 sessing a short canal, and ought never to have been separ- 

 ated. 



In the genus CERITIIIUM, 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. TROCHUS. This is a very natural genus in the Lin- 

 naean 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 SOLA- 

 RIUM, characterized by the internal spiral edge of the um- 

 bilicus being crenulated. Another species, the T. labio, is 

 the type of the genus MONODONTA, 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 Linnaeus 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 . TURBO. This very extensive genus has been greatly 



