222 ARCADiE. 



elongated, rather more tapering behind, and has the base of 

 the anterior side somewhat more prominent, in consequence 

 of the less abrupt slope of the front margin. It is a solid, and 

 occasionally (for its size) even a heavy shell, never rayed with 

 coloured streaks, but covered with an epidermis of a dirty 

 or greenish oil colour. The surface, except upon the dorsal 

 areas, is closely and finely decussated by raised radiating 

 striuke and rather irregular elevated concentric wrinkles ; 

 which latter are most perceptible towards the base and 

 sides, being sometimes almost obsolete upon the central 

 disk. The lunule is large, pouting, and well defined by 

 that abrupt cessation of convexity, which preceding the 

 retusion that bounds it, more or less sharply angulates the 

 shell. The hinder dorsal area is rather flattened, and is 

 transversely corrugated by rather closely disposed pliciform 

 wrinkles, which are not at all oblique, and are always more 

 or less undulated. Similar, but more interrupted or even 

 subgranose ones, usually adorn the lunule likewise. The 

 umbones are moderately prominent, and are placed at 

 nearly one-third the distance from the anterior extremity ; 

 they are frequently eroded in the adult examples. The 

 internal nacre is pearly white ; the margin crenulated 

 within ; the cartilage small ; the hinge-margin by no 

 means broad, and furnished with about twelve teeth in 

 front and twenty-four behind, none of which are especially 

 elongated. This proportion of front and hinder teeth 

 appears to be very general in the Nuculee. 



For its genus, it must be regarded as a large shell, one 

 of our examples measuring five-eighths of an inch in 

 breadth, and all but four-fifths of an inch in length. 



The animal appears to resemble closely that of nucleus. 



This fine and very distinct species was announced as 

 British by Mr. Jeffreys, in the nineteenth volume of the 



