CRENELLA. 203 



rather thin ; those from Newfoundland, however, are con- 

 siderably more solid. The epidermis, which entirely clothes 

 the surface, and which shines with a resinous lustre, varies 

 in colour according to the age of the specimen ; in the young 

 it is tawny, with an olivaceous tinge, changing into green 

 towards the front of the shell ; old specimens are of an 

 almost uniform pitchy brown ; intermediate stages of an 

 olive green in front, and chestnut or tar-coloured posteriorly. 

 The valves are shallow, especially in front, and are much 

 more compressed below than those of the true discors. The 

 general inclination of the ventral edge is retuse in the adult, 

 and nearly straight in the young, there being no decided 

 bulging out of outline in the centre, nor any marked pos- 

 terior lobation. The hinder dorsal edge is nearly recti- 

 linear, only very moderately elevated, and so perfectly 

 free from angulation at its junction with the hinder margin, 

 that its point of union is almost imperceptible ; the latter 

 slopes gently downward in a long arcuated sweep from the 

 highest point, (which is usually either midway from the 

 front extremity, or else from the beaks,) so that the chief 

 swell is below the middle of the posterior side, which, as 

 well as the anterior, is w ell rounded. There is no abrupt 

 depression of surface preceding the posterior compartment, 

 which in truth is little raised above the rest of the area, 

 and almost comprises two-thirds of the entire superficies : 

 this portion is occupied by most numerous elevated radi- 

 ating strise. These, although crowded, are distinctly se- 

 parated from each other, and particularly towards the 

 upper or dorsal edge, are distinctly crossed, as well as their 

 interstices, by fine raised concentric striula?, wdiich subgra- 

 nularly crenate (and occasionally even imbricate in a slight 

 degree) the radiating ones ; the latter do not suddenly 

 cease in front, but gradually diminish in elevation and 



