318 CYPRINID^. 



B. Smootli or slightly striated. 



3. A. triangularis *, Montagu. 



Mactra triangularis, Mont. Test. Brit. p. 99, t. 3. f. 5. A. triangularis, 

 F. &H. i. p. 467, pl.xxx. f.4. 



Shell forming almost an isosceles triangle, and more or less 

 equilateral, with a somewhat oblique outline, compressed and 

 sloping gradually from the umbonal part to the margins, like 

 a sharp wedge, thick for its size, rather glossy : sculpture, 

 irregular lines of growth, or a few very slight and indistinct 

 concentric ridges : colour white beneath the epidermis, which 

 is of various hues from pale yellow or orange to purplish - 

 brown or chocolate, rarely milk-white, often marked with 

 streaks of a darker tint, which radiate from the direction of 

 the beaks towards the front, where and on the posterior side 

 they are chiefly conspicuous ; the epidermis appears under a 

 high power to be microscopically punctured all over, but not 

 in rows : margins rounded in front, with an oblique curve to 

 the posterior angle, more or less straight on the anterior side, 

 and inclined to straight or but slightly curved on the posterior 

 side : beaks extremely prominent but blunt, almost central, 

 recurved a little towards the anterior side : lunule heart- 

 shaped and deep : corselet slight and short : ligament very short 

 and protuberant, of a yellow or brown tinge, according to the 

 colour of the epidermis, partly sheathed in a cardinal groove : 

 hinge-line acute-angled : hinge-plate extremely thick and 

 broad, occupying scarcely one-third of the circumference : 

 teeth as in the other species, but the chief cardinals are much 

 stronger in proportion, and the third or smallest in each valve 

 is barely perceptible, the laterals being fine and ridge-like : 

 inside polished and nacreous, sometimes faintly striated length- 

 wise ; margin either thickened and closely denticulated, or 

 else bevelled and quite smooth : pallial and muscular scars 

 distinct, especially the latter. L. 0*125. B. 0-125. 



Habitat : Local, but gregarious, on all our coasts 

 from the northern extremity of Shetland to the Channel 

 Isles, in sand, at depths of from 3 to 60 fathoms ; it is 

 remarkably abundant at Lewis in the outer Hebrides 

 and at Guernsey. It occurs, but not commonly, in all 

 our upper tertiaries and associated with high-northern 



* Triangular. 



