172 ARCID^. 



aspect : colour yellowish- white, faintly tinged with brown : 

 epidermis laminar, thicker towards the margins, and when 

 fresh forming a line of short hairs on each of the longitudinal 

 striae : margins rounded on all sides except the dorsal or 

 hinge-line ; anterior margin only half the depth of the pos- 

 terior one ; ventral margin slightly indented by the byssal 

 chink : beaks not widely separated, small but prominent and a 

 little recurved : ligament reddish-brown, slight and never per- 

 fect, composed of numerous fine threads, which cross the de- 

 pressed and narrow area at the back, and leave their impress 

 in the shape of minute striae : hinge-line quite straight, and 

 forming almost a right angle at each end, occupying nearly the 

 whole breadth of the shell : hinge-plate narrow in the middle 

 and widening towards each end, so as to afford a broad angu- 

 lar space for the reception of the teeth on either side : teeth 

 3 or 4 on the anterior side, and 4 or 5 on the posterior side, 

 indistinctly and irregularly notched on their outer edges, set 

 more or less obliquely and sometimes nearly parallel with the 

 hinge-line ; besides these teeth, and on that part of the hinge- 

 plate which lies between the two rows, is a series of minute 

 crenulations (like the ordinary teeth in A. lactea and allied 

 species), which cross the hinge-plate and lie nearly at a right 

 angle with the side teeth : inside porcellanous and somewhat 

 nacreous, obscurely marked by remote longitudinal striae ; mar- 

 gin often indistinctly notched, especially at the sides : pallial 

 scar slightly flexuous : muscular scars very large and conspi- 

 cuous. L. 0-175. B. 0-2. 



HABITAT : The Hebrides and Shetland Isles, from 35 

 to 90 fathoms, in muddy and sandy gravel. Mr. M f An- 

 drew has dredged it off Cape Clear in 60 fathoms, 

 and Capt. Hoskyn off the west coast of Ireland in 100 

 fathoms. It is an abundant fossil in the Coralline Crag 

 at Sutton. According to Scacchi and Philippi it like- 

 wise occurs in the upper tertiaries of the south of Italy ; 

 and Nyst has recorded it from a similar formation in 

 Belgium. In the Arctic seas it attains a remarkably 

 large size. Dr. Wallicli took, at a depth of 108 fathoms, 

 on the east coast of Greenland, a specimen whose di- 

 mensions nearly equal those of A. glacialis. Speci- 



