84 CONTRIBUTIONS 



These folds terminate posteriorly very abruptly at the first 

 rib, of which there are three on each valve, passing from 

 the beak to the posterior margin. The centre rib is the 

 largest, and disposed to be imbricate. A remarkable callus 

 is placed directly in the channel of the posterior part of the 

 cavity of the shell. It is disposed to be polished inside and 

 out. 



JV. pulcherrima. Plate 3. Fig. 63. 



Description. Shell an obtuse angled triangle, very trans- 

 verse, compressed, nearly equilateral, concentrically folded; 

 folds rather sharp, on the umbonial slope inflected ; sub- 

 stance of the shell thin ; beaks angular ; lunule lanceolate ; 

 escutcheon linear; anterior and posterior series of teeth 

 both incurved ; teeth arched, diminishing in size towards 

 the beak ; fosset small, subtriangular ; cicatrices impercep- 

 tible ; cavity of the shell very shallow, showing the exte- 

 rior folds and carina of the unbonial slope; margin entire ; 

 nacre not pearly. 



Diam Length .1, Breadth .2, of an inch. 



Observations. A single valve only, the right one of this 

 interesting little species, has come into my possession. The 

 deep folds which cover the whole valve, and its angular 

 form highly distinguish it from any described species with 

 which I am acquainted. 



