CRENELLA. 199 



versally claimed by our own authors for the shell we are 

 about to describe. The all but simultaneous observation 

 of its distinctiveness by three conchologists has caused the 

 imposition of three new names upon this long known 

 mussel. The valves are thin, fragile, obliquely oval or 

 rather subtrapeziform in figure, and greatly swollen ; the 

 swell is chiefly manifest upon the umbonal ridge, which is, 

 however, rounded and not carinated, and is not followed 

 by any retusion of surface, or any abrupt compression. 

 The exterior, which is covered with a thin shining trans- 

 parent epidermis, varying from greenish oil-colour to a 

 clear light brilliant green, is more or less painted with 

 minute linear angulations of a liver-colour upon a pale or 

 whitish ground. It is divided into three compartments, of 

 which the hinder is about equal to the other two united, 

 its front limit usually defined in the adult by a slight re- 

 tusion, that almost diagonally runs across the shell. The 

 anterior space, which is the smallest of the three, is radiated 

 with at least a dozen most closely set costellse or small 

 ribs, whose surfaces are obtuse and but little convex, 

 which are but little raised above their narrow interstices, 

 and which, although not really crenulated, often appear so 

 from the epidermidal wrinkles. The middle space is 

 smooth, and has the shape of a curvilinear triangle. The 

 hinder or terminal compartment, which commences a little 

 before the umbonal fold, is radiated similarly to the 

 front area, but has its more numerous costellse so much 

 more crowded that it should be rather termed striato- 

 sulcated than ribbed. The upper and lower margins of 

 the shell run nearly parallel to each other, and are 

 straightish in their general outline ; the ventral edge, how- 

 ever, swells out a little in the centre of the middle com- 

 partment, and appears subretuse at its extremities ; it 



