AtACTRA. 423 



white variegated by longitudinal rays of light reddish-brown, 

 which are often confluent, or arranged in 'sets ; purplish at the 

 beaks ; lines of growth represented by white bands or zones : 

 epidermis yellowish-brown, laminar, and glistening, at the pos- 

 terior end fibrous and frequently ramified or interlacing, so 

 as to resemble some of the sessile kinds of Tubulariidw : 

 margins regularly curved in front, and obtusely angular at 

 both ends, with a slightly rounded slope or deflexion on each 

 of the dorsal sides from the beak to the terminal angles ; the 

 anterior and posterior sides are somewhat angulated, the latter 

 more distinctly : leaks sharp, triangular, and incurved, turned 

 a little towards the anterior side ; umbones swollen and pro- 

 minent, but not much projecting : ligament short and slight, 

 yellowish-brown: cartilage triangular, compact, reddish-brown 

 or chestnut, placed obliquely as in the other species : hinge- 

 line curved : hinge-plate very broad, strong, and flexuous in 

 consequence of the projection of the cartilage-pit and adjacent 

 part : teeth, in the right valve a short, laminar, and erect car- 

 dinal, nearly parallel with the hinge-line, and another of the 

 same description and height, placed at a right angle to the 

 first, and forming one of the side-walls of the cartilage-pit ; 

 both these lie directly under the beak and on the anterior side ; 

 laterals two on each side, thin, erect, laminar, elongated, quite 

 smooth and polished ; the outer are shorter and smaller than 

 the inner pair; in the left valve are two erect and laminar 

 cardinals of equal size and height, united throughout at the 

 back and diverging at a right angle, like the flaps of a single 

 folding screen ; this valve has also a thin, erect, laminar and 

 elongated lateral on each side, which are higher in the middle 

 and curve gradually to their bases : inside white, marked with 

 clear specks, and minutely freckled, slightly striated in a lon- 

 gitudinal direction: pallial scar distinct, with a very short 

 sinus representing an arc of three -fourths of a circle : muscular 

 scars also distinct, irregularly oval and nearly equal-sized. 

 L. 1-5. B. 2. 



Var. dnerea. Shell rather more solid, ashy-grey or cream- 

 colour, without rays. M. cinerea, Montagu, Test. Brit. Suppl. 

 p. 35. 



HABITAT : From the verge of low-water mark at 

 spring- tides to 18 fathoms (Goodwick Bay, Pembroke- 

 shire), in sandy tracts ; and the variety also is widely 

 distributed. This species occurs, but not commonly, 



