TO GEOLOGY. 199 
of this curious and interesting species, has to be made from 
a single imperfect valve. It is eminently distinguished by 
its carina and broad posterior slope, from all the species 
described in this memoir. It has some resemblance to 
Sowerby’s figure of delioidea ; and Lamarck’s description 
of that shell, which is from Grignon, answers, in most of 
its characters, to ours. It is to be regretted that the pos- 
terior series of teeth should have been so much fractured 
as to present only two teeth ; these are the superior ones. 
NM. plana. Plate 6. Fig. 213. 
Description. Shell an obtuse triangle, very transverse, 
very much compressed, concentrically striate, and trans- 
versely folded on the side, sulcate on the umbonial slope ; 
substance of the shell thin; beaks obtusely angular, 
pointed ; lunule and escutcheon both nearly linear ; ante- 
rior series of teeth straight ; posterior series of teeth in- 
curved ; teeth small, numerous, angular; fosset large, 
subtriangular ; cicatrices imperceptible ; cavity of the shell 
very flat, showing the transverse folds ; nacre not pearly. 
Diam..... Length.... Breadth.... 
Observations. A single and imperfect valve only of this 
species has come under my notice. There are, however, 
characters enough remaining, to satisfy me of its being 
different from those before described. It most resembles 
the pulcherrima.* 'That shell has not, however, the re- 
markable folds and striz of this species. These strie, on 
* See page 84. 
