TO GEOLOGY. 85 
N. plicata. Plate 3. Fig. 64. 
Description. Shell elliptically transverse, rather com- 
pressed, inequilateral, concentrically and minutely folded, 
rounded before, produced and truncate behind, furnished on 
the posterior slope with six ribs—on the anterior part with 
two curved furrows; substance of the shell thin; lunule 
lanceolate ; escutcheon long, lanceolate ; anterior series of 
teeth arched, posterior series incurved ; teeth angular, di- 
minishing in size towards the beak; fosset small, angular ; 
cicatrices imperceptible ; cavity of the shell very shallow, 
having its channel interrupted by a callus; margin entire ; 
nacre not pearly. 
Diam. .1, Length .2, Breadth .5, of an inch. 
Observations. 'This species resembles in outline the 
Brogniarti and media. It differs from the first essentially in 
the closeness and regularity of the folds, and from the last 
in having the folds to continue over its anterior portion. 
It differs from both in the position of its beaks, which are 
placed so much towards the anterior part as to render the 
shell very inequilateral. The ribs too are more sharp. 
The furrow on the anterior part of the valve of this species 
is not found in either of the above described species. 
This fine genus seems to pervade nearly all the import- 
ant formations, from the Carboniferous Limestone to the 
newest of the Supercretacious Group. The . Palme* 
* Sowerby’s Min. Conch. vol. 5. 
