156 ARCIDiE. 



filaments : tubes united for more than half their length, 

 considerably produced, slender and smooth ; the incurrent 

 (or branchial) tube is shorter than the excurrent (or anal) 

 tube, and the latter has a square orifice, with finely pointed 

 angles : foot oblong, compressed, white, deeply grooved, and 

 capable of being expanded into a creeping-disk with notched 

 margins. 



Shell inequilateral, sloping from the beak with an oblique 

 curve to the posterior extremity, which somewhat resembles 

 a duck's bill, compressed, rather solid and opaque, scarcely 

 glossy : sculpture, about 30 transverse laminar ribs which 

 vary very much in strength and compactness, besides extremely 

 fine glittering and prismatic lines, which radiate from the 

 beaks, but do not usually extend to the front margin unless 

 in very young shells ; and there are also two ridges, which 

 diverge from the beak in each valve, and embrace the trun- 

 cated point at the posterior angle : colour pearl-white under 

 the epidermis, which is yellowish-brown and generally of a 

 dull hue : margins rounded at the anterior side, more or less 

 curved in front, and elongated at the posterior side to a blunt, 

 upturned, and truncate point, where the valves slightly gape : 

 beaks small, rounded, nearly straight, destitute of transverse 

 ribs, and glossy : lunule lance-shaped, ribless, depressed, with 

 slightly prominent edges or lips, and defined by the inner 

 ridge of the posterior slope : ligament slight : cartilage oval 

 and small, contained in an oblique pit : hinge-line curved on 

 the anterior side of the dorsal area, and inflected on the other 

 side, occupying about two-fifths of the circumference : hinge- 

 plate grooved, and increasing in width in proportion to the 

 growth of the shell: teeth vault-like or concave, slightly 

 curved outwards, higher in the centre of each row, and dimi- 

 nishing in size towards either end ; there are about 16 on the 

 anterior side, and about 20 on the posterior side : inside por- 

 cellanous and glossy, somewhat thickened near the margin, 

 which is smooth, with a distinct ridge running down the middle 

 of the beak -like extension of the posterior side : pallial and 

 muscular scars well marked. L. 0'3. B. 0-55. 



Var. brevirostns. Shell smaller, longer in proportion to its 

 breadth, with the posterior extension much shorter, more con- 

 vex or tumid : ribs finer and more crowded. 



Habitat : Everywhere on our northern coasts, in 

 muddy gravel and sand, from 20 to 90 fathoms. Captain 



