IS0CARDIA. .299 



orifices : gills free, concealed between the mantle and the rest 

 of the body : foot very muscular, orangecolour. 



SHELL almost spherical, solid when adult, of a dull hue : 

 sculpture, faint concentric striae, with occasional deeper lines 

 of growth : colour yellowish- white, variegated in the young by 

 zigzag streaks of reddish-brown : epidermis chestnutcolour, 

 rather thick, covered with extremely numerous rows of very 

 short fine bristles, which are of a darker colour, and radiate 

 from the beak to the margins, except on the upper part of the 

 anterior side where the cilia are wanting : margins curved in 

 front, sloping upwards on the anterior side to a blunt angle 

 formed by its junction with the dorsal margin, obliquely 

 truncate at the posterior side, which is indistinctly sinuous ; 

 dorsal margin projecting : beaks excentric, gracefully recurved 

 and making an incomplete whorl : lunule forming a broad ex- 

 cavation below the beaks, and enlarging rapidly during the 

 progress of growth : ligament strong and raised, dark reddish- 

 brown, following the bend of the posterior dorsal margin : 

 hinge-line curved : hinge-plate thick and broad, reflected out- 

 wards, occupying rather more than one-third of the circum- 

 ference : teeth, in the right valve two cardinals, the outer of 

 which is laminar and nearly parallel to the hinge-line, and the 

 inner is shorter, very much thicker, and cloven obliquely; 

 between the latter tooth and the inside of the shell is a deep 

 socket for the reception of a similar tooth in the opposite 

 valve ; lateral tooth short and triangular ; the left valve has 

 two cardinals corresponding with those in the other valve, 

 except that the outer one is cloven obliquely, and the socket 

 is placed on the inside of the thicker tooth ; the lateral tooth 

 in this valve is longer than that in the right valve, but not so 

 high: inside chalky, with sometimes a faint tinge of flesh - 

 colour, irregularly furrowed lengthwise, apparently in con- 

 sequence of an unequal secretion by the mantle : pallial scar 

 broad and shining : muscular scars rather large, that on the 

 anterior side being remarkably deep. L. 4. B. 3-8. 



HABITAT : Muddy ground in 4-40 fathoms on the 

 coasts of Devon and Cornwall, east and south of Ireland, 

 and west of Scotland. Laskey is said to have taken it 

 off St. AbVs Head. It is not generally distributed, 

 although tolerably plentiful in certain places. Searles 

 Wood has recorded it as a fossil of the Red and Coral- 



