416 MACTRIDJS. 



below them a bright orange line often encircles the siphonal 

 sheath; a conical valve is now and then seen to protrude 

 from the opening of the excretal tube : gills varying in colour 

 from light orange to reddish-brown ; upper pair larger than 

 the other, obliquely and very finely pectinated: palps long, 

 narrow, pointed, brown of different shades, more distinctly 

 striated than the gills, particularly on the inner surfaces : foot 

 large, fleshy, and white. 



Shell representing in form an isosceles triangle, convex, 

 usually solid and thick, opaque, somewhat but not very glossy, 

 owing to the continual abrasion of the surface by the sand in 

 which it is buried : sculpture, numerous slight and irregular con- 

 centric striae, which often become laminar and almost rib-like 

 in front and at the sides, and always at the dorsal margin ; 

 lines of growth strongly marked ; on the anterior and ventral 

 sides in some specimens may also be observed a few slight 

 longitudinal raised lines which radiate from the beak : colour 

 pale yellowish-white, sometimes stained with ochre from a 

 sediment deposited in shallow or tidal waters: epidermis 

 yellowish-brown, silky in the young and coarse in the adult, 

 being in the latter state more persistent at the sides and 

 towards the edges : margins curved in front, obtusely angular 

 at each end, but more pointed at the posterior side, with a 

 rounded slope from the beak to either extremity ; the sides 

 (especially the posterior) are more or less angulated : beaks 

 very small and rather blunt, incurved, turning a little towards 

 the anterior side ; umbones more or less tumid and projecting : 

 ligament short and slight, yellowish-brown: cartilage trian- 

 gular, compact and strong, golden-yellow, placed obliquely in 

 a direction opposite to that of the beaks and immediately 

 under the ligament : hinge-line obtusely angular : hinge-plate 

 extremely broad, thick, and flexuous, the cartilage-pit project- 

 ing in the middle : teeth, in the right valve a short laminar 

 and erect cardinal on the anterior side, nearly parallel with the 

 hinge-line, and a much slighter and scarcely raised cardinal 

 forming one of the sides of the cartilage-pit ; laterals in this 

 valve two on each side, the inner surfaces of which are strongly 

 pectinated or grooved in a perpendicular direction ; the left 

 valve has two short cardinals, also on the anterior side of the 

 cartilage-pit, united at the base or point of the fork, and of 

 equal height and length ; laterals in that valve one on each 

 side, deeply grooved like the laterals in the right valve, but on 

 both sides and not merely on their inner surfaces : inside more 



