248 LUCINID^E. 



gular lines of growth and microscopical frost-like markings : 

 colour white : epidermis filmy and inconspicuous : margins ob- 

 liquely truncate on the anterior side, and forming nearly a 

 right angle with the dorsal margin, which is almost straight or 

 slightly incurved, rounded in front, and flexuous on the pos- 

 terior side, which is deeply indented by a wide furrow or 

 groove running from the beaks at an angle of about 30 degrees 

 and gradually enlarging outwards, as well as by another smaller 

 and parallel furrow close to the ligamental area : beaks small 

 and pointed, but not prominent, much recurved, almost con- 

 tiguous, a little nearer to the anterior side : lunule rather deep 

 and heartshaped : corselet prominent, well defined by the upper 

 furrow, and forming two sharp and pouting lips : ligament 

 rather long, yellowish-brown or light horncolour, not pro- 

 jecting beyond the lips of the corselet, but distinctly visible 

 outside ; ligamental groove narrow and shallow : hinge-line 

 obtusely angular, occupying not one-fourth of the circumfe- 

 rence : hinge-plate rather broad and thick, nearly flat, almost 

 all of it lying on the posterior side ; immediately below the 

 beaks it is indented in the right valve and slightly reflected in 

 the left, which gives that valve the appearance of having an 

 indistinct or obscure cardinal tooth : inside highly glossj T and 

 iridescent, sometimes slightly striated lengthwise ; margin thin 

 and plain : pallial scar broken in a zigzag line, in conse- 

 quence probably of a corresponding structure in the edge of 

 the mantle : muscular scars large. L. 0375. B. 0*35. 



Yar. polygona. Rather longer in proportion to the breadth, 

 marked by three or four longitudinal ridges, giving the shell 

 an angular form, the grooves on the posterior side being much 

 deeper than usual. 



Monstr. Having a longitudinal groove down the middle, 

 which is longer and deeper in the left valve than in the right. 



Habitat : Gregarious in soft mud and sand on all 

 the British coasts, from Shetland to the Channel Isles, 

 in 3-87 fathoms. The variety is from the deepest water 

 on the " outer haf," about forty miles east of the 

 Whalsey Skerries, and the monstrosity from Falmouth 

 and Tenby. This species is rather plentiful in the 

 " alluvial" deposits at Belfast (Hyndman and Grainger); 

 Clyde beds (Smith); Coralline Crag, Sutton (S. Wood). 



