196 pectenidj:. 



Inhabits St. George's Bank. Gulf St. Lawrence (^Mighels} ; 

 Northwest coast of Greenland {Hai/cs}. 



This very strongly-marked species seems to have been hitherto 

 imdescribed. It is closely allied to Modiolaria discors, but is 

 smaller, stronger, lighter colored, and entirely covered with ribs. 

 Of five or six specimens which I have seen, all apparently mature 

 shells, the largest was of the size given above. Modiola arctica, 

 Leach, is a smaller and more rounded shell, and the ribs are much 

 more numerous. 



Family PECTENID^, Lamarck. 



Ligament interior or half interior. Shell in general regular, com- 

 pact, not foliated. 



Genus PECTEIV, Brug. 1789. 



Shell rounded, inequivalve, eared ; superior margin straight ; 

 beaks contiguous. Hinge toothless, with a triangular internal pit 

 for the cartilao-e. 



"O^ 



Pecten tenuicostatus. 



Shell orbicular, inequivalve, upper valve more convex than the lower ; exterior 

 surface everywhere marked with closely-arranged, radiating lines ; interior sur- 

 face without ribs ; ears small and equal. 



Pecten MarjeJlanicus, Conrad, Amer. Mar. Conch, pi. 1, fig. 1. — Gould, Inv. 1.32. — 

 De Kay, N. Y. Moll. 173, pi. 11, fig. 207. — Stimpson, Shells of New England, 8. 



Pecten tenuicostatus, Mighels and Adams, Bost. Journ. Nat. Hist. iv. 41, pi. 4, fig. 7; 

 Tr. i. 49 (1841). — Stijipson, Smith. List. Check Lists (1860), 2. 



Shell orbicular, rather higher than long, thin and translucent 

 when young, thick, strong, and opaque when mature, equilateral, 

 inequivalve, the lower valve being nearly flat, and not attaining the 

 edge of the upper valve by an eighth of an inch or more ; upper 

 valve moderately convex ; valves widely gaping near the hinge, 

 surface everywhere sculptured with radiating, punctured lines, or 

 grooves, about half as wide as the spaces between them, somewhat 

 zigzag in their course ; these lines are crossed by closely-arranged 

 lines of growth, which, on the convex valve, are scalloped or vaulted 

 over the radiating lines ; flattened valve white, convex valve dingy 

 reddish-brown or flesh-colored. Hinge-margin narrow, straight, ears 

 equal, the notch in the lower valve rounded and shallow. Interior 



