TO GEOLOGY. 199 



of this curious and interesting species, has to he 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 deltoidea ; 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. 



N.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. 



Diara 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 pukherrima.* That shell has not, however, the re 

 markable folds and strise of this species. These strise, on 



* See page 84, 



