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 ( c Beretning om en 

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

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

 different species. 



3. A. lac'tea *, Linne. 



A. lactea, Linn. Sjst. 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 : bgssus 

 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: byssal sinus some- 



* Milk-white. 



i 5 



