TO GEOLOGY. 83 



umbonial slope. As they approach the anterior margin 

 they become obsolete, but again increase, and are deflected 

 after passing an oblique impressed line, passing from the 

 beaks to the anterior basal margin. Posterior to the um- 

 bonial slope the folds are also there deflected, and become 

 smaller. In the furrows very minute transverse striae may 

 be observed. The lunule, escutcheon and furrows between 

 the ribs are slightly striate. The crenulation of the mar- 

 gin is very minute, and can only be observed on very per- 

 fect specimens. 



JV. media. Plate 3. Fig. 62. 



Description. Shell elliptically transverse, somewhat in- 

 flated ; rounded before, produced and truncate behind, equi- 

 lateral, furnished with fine concentrical folds in the middle, 

 and three ribs on each valve behind ; anterior part smooth ; 

 substance of the shell thin ; lunule none ; beaks very small, 

 pointed ; anterior series of teeth arched posterior series in- 

 flected ; teeth small, pointed, angular, diminishing in size 

 towards the beak ; fosset scarcely perceptible ; cicatrices ob- 

 solete ; cavity of the shell rather shallow, having its channel 

 interrupted by a callus ; margin entire ; nacre not pearly. 



Diam. .1, Length 3-20ths, Breadth 7-20ths, of an inch. 



Observations. This beautiful little species in outline is like 

 the last described, but differs much in many of its charac- 

 ters. The folds are minute and very remarkably regular. 

 Becoming obsolete as they approach the anterior margin, 

 they there vanish altogether as if erased by attrition. 



