442 MACTRlDiE. 



4. S. te'nuis*, Montagu. 



Mactra tenuis, Mont. Test. Brit. Suppl. p. 572, t. 17. f. 7. Syndosmya 

 tenuis, F. & H. i. p. 323, pi. xvii. f. 11. 



Bodi white : mantle having its edges delicately fringed : 

 tubes cylindrical and narrow ; the alimentary one capable of 

 being extended twice the transverse admeasurement of the 

 shell, the other usually recurved and shorter by one-third ; 

 orifices apparently plain: foot sernitransparent, extremely 

 flexible. 



Shell rather triangular than oval, moderately convex, 

 nearly equilateral, thin, opaque, generally not glossy although 

 iridescent : sculpture, fine and close-set concentric strias, the 

 interstices of which are traversed by twice as many microsco- 

 pical longitudinal strise ; lines of growth irregular but conspi- 

 cuous : colour ashy-white : epidermis laminar, light yellowish- 

 brown : margins gently curved in front, rounded on the anterior 

 side, truncate and nearly straight on the posterior side, which 

 is bluntly angulated and gapes a little ; dorsal margins straight, 

 and forming an angle of about 60 degrees with the beak as 

 an apex, slightly excavated or depressed on both sides : beaks 

 small and calyciform, inclining a little to the posterior side ; 

 umbones projecting: ligament minute and narrow, dark horn- 

 colour: cartilage curved, yellowish-brown: hinge-line more rect- 

 angular than obtuse- angled : hinge-plate of moderate strength 

 and thickness, occupying in the right valve about one-fourth, 

 but in the left not one-tenth of the circumference : teeth, in 

 the right valve two small erect and parallel cardinals, set (as 

 usual) transversely to the hinge, besides one short double 

 laminar lateral on the anterior side and a similar but single 

 lateral on the posterior side ; these laterals are triangular and 

 rather abruptly truncate at their further extremities ; the 

 left valve has only one cardinal, similar to those in the right 

 valve, and no lateral tooth (unless an obscure sunken fold or 

 socket on each side of the hinge can be so called) : inside 

 somewhat nacreous, but not polished, indistinctly striated 

 lengthwise, microscopically and closely freckled ; margin be- 

 velled and sharp : pallial scar well defined ; sinus large and 

 triangular, with its blunt apex below the beak, and its base 

 parallel with the front margin : muscular scars rather deep, 

 anterior oval, posterior trapezoidal. L. 0*3. B. 0-4. 



* Thin. 



