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. 
N. pulcherrima. Plate 3. Fig. 65. 
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... <7 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. 
