156 



being crowded together under the same genus ; and the 

 fossil shells which were subsequently discovered differed so 

 much from those which were before known, as to render an 

 an augmentation of the number of the genera absolutely 

 necessary. This work was commenced by M. Bruguiere ; 

 and has been since most successfully conducted by Le 

 Chev. de Lamarck, to whom we are much indebted for 

 making a considerable augmentation of the number of ge- 

 nera, and for suggesting several other important changes in 

 the classification. 



In this part of the present work it is proposed to give 

 the most prominent characters of the known genera, with 

 sketches illustrative of their distinctive differences. 



Univalves. 



GCNUS 1. Planospirites. A flat univalve, nearly cir- 

 cular, with a cord-like ridge passing from the edge on to 

 the inferior surface, and curving backwards in a spiral 

 direction. Fossil, St. Peter's, Maestricht. PI. iii. fig. 1. 



2. Oscana. An oval univalve, slightly vaulted ; no spire, 

 semitransparent, and nearly coriaceous. Recent. PL iii. 

 fig. 2. 



3. Testacella. An obliquely conical univalve; apex 

 turned ; opening oval ; the left edge turned inwards. lie- 

 cent. PI. iii. fig. 3. 



Grows on the caudal termination of an animal generally 

 resembling a slug. 



4. Patella. A shield formed, subconical univalve ; no 

 spire, and without perforation of the disk, or marginal 

 fissure.* Recent and fossil. PI. iii. fig. 4. 



* Patella mitrata, Lin. and P. cornucopia and dilatata, Lam. 

 have the peculiar property of being attached to an operculum, or 

 undervalve, by a muscle, the mark of attachment of which, espe- 

 cially on the lower valve, bears somewhat of the figure of a horse- 

 shoe ; this valve being attached by the other side to other bodies. 



