ARCA. 177 



and from the latter in its smaller size and coarser sculp- 

 ture, and in having only half the number of teeth. The 

 longitudinal furrow or indentation in the middle is also 

 a peculiar character of the present species. Norwegian 

 specimens are twice the size of ours, and these last are 

 larger than Forbes's specimens. 



It is the A. Korenii of Danielssen (' Beretning om en 

 Zoologisk Reise/ 1859) and A. lactea of Malm. The 

 A. obliqua of Ileeve is a West- African shell, and a very 

 different species. 



tf'PL 3. A. LAC'TEA *, \Linne3 ft . 3 c 



A. lactea, Linn. Syst. Nat. p. 1141 ; F. & H. ii. p. 238, pi. xlvi. f. 1-3. 



BODY white : mantle having its border finely notched, and 

 pale-red towards the middle of the dorsal area; the under 

 .surface of the ventral line is marked with irregular flake- 

 brown blotches on a pale-yellow ground, and the upper surface 

 is marked for some little depth with a sand-like rusty-brown 

 belt and a darker interrupted line nearer the margin : gills 

 symmetrical and equal in size, very thin and gradually taper- 

 ing, pale yellow : foot extensile, fleshy, and pure white : byssus 

 short, horny, composed of several leaf-like threads. 



SHELL varying in shape from rhomboidal to triangular, 

 tumid, solid, of a dull hue : sculpture, numerous and fine lon- 

 gitudinal ribs, which become fewer and stronger at the sides, 

 besides slighter and rather more numerous transverse ribs, 

 which cross the other, making the front surface appear reticu- 

 lated, and forming rows of minute tubercles at the point of inter- 

 section ; sometimes a few smaller and intermediate longitudinal 

 ribs are perceptible in front: colour yellowish- white: epidermis 

 brown, laminar and downy, thicker and forming towards the 

 front and sides longer threads, which occasionally resemble 

 short bristles : margins rounded at the anterior side, straight 

 or nearly so in front, obtusely wedge-shaped and somewhat 

 truncate on the posterior side, with a blunt keel or ridge sepa- 

 rating that side from the rest of the shell, and which is very 

 distinct and sharp on the umbonal area : lyssal sinus some- 



* Milk-white. 



i 5 



