176 ARCID^!. 



compressed in the middle and indistinctly farrowed by a lon- 

 gitudinal groove, which runs from the beak to the front margin, 

 convex, rather solid, scarcely glossy : sculpture, numerous fine 

 and rounded ribs, radiating from the umbo, and equally nume- 

 rous but laminar transverse striae which are raised or imbricated 

 over the longitudinal ribs a reticulated appearance resulting 

 from their crossing each other : colour milk-white : epidermis 

 thin and silky : margins rounded on all sides except behind, 

 with a slight inflection in front, sloping abruptly on the 

 anterior side, and wedge-shaped on the posterior side ; dorsal 

 angle well developed : beaks small, blunt, and glossy, placed 

 near the anterior margin and close to the hinge-line : ligament 

 yellowish, slight, and narrow : hinge-line nearly straight, 

 occupying three-fourths of the shell's breadth, gently curved : 

 teeth arranged in two rows, that on the anterior side consist- 

 ing of 4 or 5, and the other of 10 or 12, set nearly at a right 

 angle to the hinge-line, but slightly diverging as they ap- 

 proach each end ; the intermediate space is smooth : inside 

 porcellanous, marked with a few longitudinal stria3, which 

 only extend to the pallial scar ; margin thickened and broad, 

 with the edges faintly crenulated : muscular scars very large 

 and conspicuous. L. 0-15. B. 0-2. 



HABITAT: 80-85 fathoms, off Unst, in shell-sand. 

 Two valves only (right and left) have been found, one 

 by myself and the other by Mr. Waller ; both are fresh 

 and perfect. It is a Scandinavian species, and has been 

 taken by Danielssen in 60-80 fathoms at Vadso, by 

 Sars in West Finmark, and by Malm in 100 fathoms 

 on the Bohuslan coast. It was discovered by Philippi 

 in newer tertiary strata at Lamato in Calabria; and 

 Mr. M' Andrew has shown me two recent valves which 

 he received from the late Professor Forbes, probably 



This interesting addition to the British mollusca is 

 intermediate between A. pectunculoides and A. lactea. 

 From the former it differs in being shorter in proportion 

 to its breadth (the shape also being more oblique), and in 

 having the beaks placed much nearer to the anterior side, 



