MACTRA. 363 



pale livid, only diversified by an obscure and partial radia- 

 tion. The pale subtriangular variety, which is much less 

 common than the more oval one, constitutes the M. cinerea 

 of Montagu. The rays, although never broad, vary in 

 relative width upon the same specimen ; there is frequently 

 a tinge of purplish liver-colour upon the umbones (not the 

 beaks, which are usually paler), and there is often a stain 

 of fawn-colour beyond the umbonal ridge, even when the 

 rest of the surface is almost devoid of colouring. Less 

 frequently the rays are fawn-coloured upon a paler ground. 

 The valves are rather thin, not quite opaque, and more 

 usually semipellucid ; they are glossy, and almost smooth, 

 being neither striated nor grooved ; when magnified, how- 

 ever, a kind of crowded concentric subimbricated wrinkle- 

 like lineolation displays itself, especially upon the umbonal 

 fold, and towards the lower margin. Recent examples are 

 covered with an epidermis of a cinereous brown, straw- 

 coloured drab, or yellowish ash-colour, which is most 

 closely and delicately wrinkled in a concentric direction. 

 The valves are rather swollen, the chief profundity being 

 at the subumbonal region, from whence it diminishes with 

 tolerable evenness on either side ; there is a slight, but evi- 

 dent flattening of surface upon both dorsal areas, which 

 are equally free from sculpture with the rest of the shell ; 

 the hinder area is the more depressed. The amount of 

 this dorsal compression mainly determines the contour of 

 the shell, which, when it is but very trifling, and the lips 

 of the suture pout, the slopes being more or less arcuated, 

 is oval ; but when more violent, so that the lips do not 

 pout, aud the slopes are comparatively rectilinear, becomes 

 subtrigonal. The front dorsal edge is, however, almost 

 invariably arched below, and its declination, though de- 

 cided, is not quite equal to the produced, and rather ab- 



