228 ARCADE. 



squarely truncated at the tip. The lunule is large, con- 

 cave (but with pouting lips), spindleshaped, and devoid of 

 all sculpture ; it is well defined by the sharply angulated 

 upper edge of the flattened umbonal fold, which likewise 

 exhibits an angulation, though a less marked one, at its 

 lower edge, where its bounds are clearly indicated by 

 the abrupt flexure of the girding lyne. The beaks which 

 are situated at about three- sevenths of the distance from 

 the rounded extremity are acute, prominent, and only 

 slightly inclined to the longer side. The inside is of 

 a polished white (not nacreous) which has almost invari- 

 ably a bluish cast ; the cartilage-pit is appressed ; there 

 are about eighteen vaulted spinous teeth on the longer 

 side, and about sixteen on the other ; the margin is quite 

 plain. 



The other (or northern) variety, generally a smaller 

 shell, being of a more convex and abbreviated shape, has 

 consequently its lower margin more arched, and its rostrum 

 more sudden information. Some very delicate and obscure 

 radiating lineoles are almost invariably perceptible ; the 

 sides are also more unequal ; and the striae, which are 

 usually finer and 'more crowded, do not quite extend to 

 either extremity. In the more characteristic examples, 

 the swell of the ventral edge is very unequal, and not 

 gradual as in the typical variety. Were these features 

 permanent in every individual, they might be probably 

 estimated as of specific importance ; but as we have 

 examples before us which unite the exact shape of the 

 previous form, to the radiating lineoles, &c, of this one, 

 we cannot venture to consider the two shells as more than 

 varieties of the same species. 



The length of a large specimen was two-thirds of an 

 inch ; its breadth was four lines and a half: the ordinary 



