SCROBICULARIA. 327 



tlielr more trigonal brethren of the Atlantic Ocean. Our 

 British examples, which all display a certain degree of tri- 

 angularity, vary in shape from elliptic to suborbicular, the 

 length being, however, always greater than the breadth. 

 The valves are subequilateral, compressed, dull, or but 

 very slightly glossy, and of an uniform squalid white, but 

 very frequently stained black by the colour of the mud in 

 which they are imbedded : they are opaque, fragile, though 

 not particularly thin, and either smooth, or marked with 

 very delicate concentric wrinkles, and more distant stages 

 of growth. The ventral margin is not much arcuated, and 

 whatever degree of ascent exists at its extremity, is almost 

 always straightish, or even at times slightly retuse, near the 

 middle. The dorsal edges vary greatly in the amount of 

 declination, but are always more or less straight, and the 

 ligamental one is generally the more sloping. The anterior 

 side is very slightly the shorter, its extremity is rounded, 

 but more regularly so below than above. The posterior 

 side is but little attenuated at its extremity, which is 

 bluntly subangulated, and situated rather below than above 

 the middle of the side, its exact elevation being dependent 

 upon the greater or lesser rise of the ventral margin poste- 

 riorward. The umbones are rather prominent ; the beaks 

 acute and incurved, not leaning to either side, nor preceded 

 by any lunule-like depression. The ligament is small and 

 narrow, but rather projecting : there is a certain degree of 

 depression at the hinder dorsal surface of the shell. There is 

 little or no elevation of the umbonal ridge, but there is usually 

 a more or less obsolete shallow groove adjacent to its site. 

 The inner surface is of a glossy white, occasionally, but 

 rarely, stained slightly with yellow : the cartilage pit is 

 triangular, and rather large ; in front of it lie the small and 

 extremely narrow primary teeth, a simple and solitary one 



