GASTEROPODA PECTINIBRANCHIATA. 267 



DOLIUM, Lam. 



Those in which projecting ribs, that follow the direction of the whorls, 

 render the margin undulated; the inferior whorl is ample and ventricose. 



HARP A, Lam. 



The Harpx are easily recognized by the projecting, transverse ribs on 

 the whorls; the last of which forms a lip on the margin. The shell is beau- 

 tiful, and the animal has a very large foot, pointed behind, and widened in 

 its anterior portion, which is distinguished by two deep emargtnations. The 

 eyes are on the sides of the tentacula, and near their base. It has neither 

 veil nor operculum. The 



PURPURA, Brug. 



Is known by its flattened columella, which is trenchant near the end op- 

 posite to the spire, and which, with the external margin, forms a canal there, 

 sunk in the shell, but not salient. The animal resembles that of a true 

 Buccinum. 



The remaining subgenera are Concholepas, Cassis, Cassidaria and Terebra. 



CERITHIUM, Brug. 



A shell with a turriculated spire; the aperture is oval, and the canal short, 

 but well marked, and reflected to the left or backwards. The animal has 

 a veil on its head, and is furnished with two separated tentacula, on the 

 side of which are the eyes, and with a round, horny operculum. Many are 

 found fossil. 



MUREX, Lin. 



This genus comprises all those shells in which there is a salient and straight 

 canal. The animal of each subgenus is furnished with a proboscis, long 

 approximated tentacula, on the external side of which are the eyes, and with 

 a horny operculum; the veil on the head is wanting, and, the length of the 

 siphon excepted, it otherwise resembles that of the Buccina. Brugiere 

 divides them into genera, which have been since subdivided by Messrs La- 

 marck and Montfort The 



MUREX, Brug. 



Includes all those which have a salient and straight canal, with varices 

 across the whorls. 



RAKELLA, Lam. 



Characterized by opposing varices, so that the shell is bordered with them 

 on both sides. Their canal is short, and their surface studded with mere 

 tubercles; margins of the aperture plicated. The 



