CONCHACEA. MOLLUSCA. ASTARTE. 81 



Shell orbicular-elliptical, rather thin, much compressed ; sur- 

 face marked with rather remote, rounded, rib-like ridges, most 

 conspicuous along the posterior slope of the disk ; covered with 

 a dark yellowish-brown epidermis ; beaks nearly central, elevated 

 and inclined forwards, with a short, lanceolate lunule in front, and 

 a longer corselet behind, both of them deeply excavated ; the 

 anterior slope from the beaks is concave for a short distance, the 

 posterior is straight, and both extremities are broadly rounded ; 

 ligament broad ; hinge margin narrow and rather feeble, and car- 

 dinal teeth slightly elevated ; on one side of them is a tooth-like 

 ridge along the margin, and on the other a groove to receive this 

 ridge ; in the right valve the groove is before, and the ridge behind, 

 the cardinal teeth ; cavity of the valves rather shallow ; muscular 

 and palleal impressions rather indistinct ; the margin, in the speci- 

 mens I have, is sharp and not crenulated. Length lj\ inch, 

 height 1 inch, breadth | inch. 



This shell, from the cabinet of Colonel Totten, was brought from 

 the Grand Bank. Of the two specimens, one is about half the size 

 of the other. 



The specimen from the Arctic seas, figured in the " Appendix to 

 Beechey's Voyage," is much larger than either of these. 



It is distinguished fi'om A. sulcata by its broadly rounded extremi- 

 ties, compressed form, delicate and short ridges, slender hinge, shorter 

 impressions each side of the beak, and its probably plain margin. 



AsTARTE QUADRANS. 



Shell triangular, slightly oblique, anterior side longest ; surface 

 smooth ; epidermis yellowish- olive ; hinge with a lateral tooth on the 

 anterior margin. 



Figure 48. 



State Coll., No. 202. Soc. Cab., No. 2331. 



Shell small, triangular, solid, nearly equilateral, side margins 

 bounded by straight lines, the anterior somewhat longer and more 

 oblique than the posterior ; basal edge very sharp and regularly 

 rounded, so that the whole shell is nearly a quadrant ; beaks 

 pointed, and not inclined to either side, generally eroded ; lunule 

 lance-shaped, slightly impressed, and a broader and longer areola 



II 



