AMPHIDESMA. 413 



Amphidesma casta neum *, Montagu. 



Donax castanea, Mont. Test. Brit. App. p. 573, t. 17. f. 2. Ervilia castanea, 

 R & H. i. p. 341, pi. xxxi. f. 5, 6. 



Shell obliquely and triangularly oval, or inclining to ob- 

 long, convex, thick, opaque in the adult and semitransparent 

 in the young, glossy : sculpture, slight and irregular concentric 

 striae, which now and then become laminar on the anterior 

 side : and in fresh specimens may also be observed numerous 

 and minute longitudinal striae, especially on the posterior side, 

 which radiate from the beaks and extend to the margins : 

 colour reddish-chestnut, of a deeper tint on the posterior side, 

 with a short chocolate streak or ray in each valve, curving 

 from the front or ventral margin to about two-thirds of the 

 distance from the beak ; this streak is narrow at the top and 

 rapidly enlarges to its base as in Donax politus ; occasionally 

 the colour is milk-white : epidermis slight, yellowish : margins 

 nearly straight or but little curved in front, rounded and semi- 

 oval at each end, sloping behind from the beaks at an obtuse 

 angle, where the dorsal margins are straight, and half as long 

 again on the posterior as on the other side : beaks veiy small, 

 blunt and calyciform, incurved, turning a little towards the 

 posterior side ; umbones rather tumid but not projecting : liga- 

 ment extremely small and caducous : cartilage thick, contained 

 in a triangular pit which shelves inwards and is enclosed by 

 strong walls or sides : hinge-line gently curved : hinge-plate 

 very strong, solid, and broad : teeth, in the right valve a large 

 erect, triangular and oblique cardinal on the anterior side, and 

 a smaller diverging one on the other side of the cartilage-pit, 

 besides a long but indistinct laminar lateral on each side ; in 

 the left valve are similar cardinals but of more equal size, and 

 the laterals are usually obliterated or obscure : inside polished 

 and glossy; margin minutely and closely crenulated: pallia! 

 scar well defined, with a rather short oval sinus which does 

 not reach half way across : muscular scars oval, the anterior 

 being the deeper and more conspicuous. L. 0-3. B. 0-5. 



Habitat : Coasts of Cornwall and Scilly Isles, in 

 20-54 fathoms, but more frequently thrown up on sandy 

 beaches ; Herm (Macculloch) ; Cork (Humphreys and 

 J. G. J.) ; dredged off Arran Isles, co. Galway, in 10-12 



* Chestnut- colour. 



