152 ARCADiE. 



of transverse indentations may be seen along cacli side of the pos- 

 terior hinge-margin. 



[The above comparisons were made with specimens sent from 

 England, none of which are now regarded as the true N. nucleus ; 

 and since two or three species liave been eliminated (iV. radiata, de- 

 cussately 7iitida, &c.) the remainder seem in no respect to differ from 

 the American shell. 



Nucula expansa. 



Shell large, ovate-triangular, tumid, dusky, chestnut-colored surface, distinctly 

 waved, and with line radiating striiE on both dorsal areas. 



Nucula Bel I of. a, Adams, Zool. Proc. (185G), p. 51. — IIaxlev, in Sown. Thcs. Conch. 



iii. 162, pi. 229, fig. 128. — Packaku, Labr. Mar. Sh. l.'3. 

 Nucula expdnsa, Rekvk, in Belcher's last Arct. Voy. 397, pi. 33, fig. 2 (1855). 



Shell large, thin, ovate triangular, tumid, without marked umbonal 

 ridges ; anterior dorsal margin nearly vertical and straight, the area 

 broad heart-shaped, depressed, bounded by an obtuse ridge and 

 marked by fine diverging stria), anterior end rounding 

 ^^^'J^ into the decidedly arcuate ventral margin ; posterior dor- 

 sal margin much longer and more oblique than the an- 

 terior, gently curved, and with no angles ; dorsal face 

 broad, somewhat flattened, with the lips of the valves 

 scarcely pouting, and with a few delicate radiating lines ; 

 surface concentrically ridged, quite coarsely so at the 

 N.T^^sa outer circles ; epidermis glossy, dark chestnut, with yel- 

 lowish zones. Interior leaden color, muscular impres- 

 sions deep, margin simple, covered by the inflected epidermis ; hinge 

 rather robust, the ligament spoon very oldique, with aliout ten teeth 

 in front and fifteen behind it. Length, eleven twentieths of an 

 inch; height, seven twentieths of an inch; breadth, six twentieths 

 of an inch. 



Dredged by Dr. Packard in fifteen fathoms at Chataque Bay, 

 Lal)rador ; Arctic Seas {Han/e/j^ ; Beechcy Island (Be/chef). 



One of the largest and finest of the genus, so remarkable for its 

 undulated surface and dark ei)idermis as to assimilate it to Corhicula. 



Nucula inflata. 



Shell trapeziform, obliquely truncated, tumid, lieaks prominent, anterior. 



Nucula inflata, Hancock, Ann. Nat. Hist. (1846), 333, pi. 5, figs. 13, 14. — Haxley, in 



SowB. Thes. Conch, iii. 162 (Nucula), pi. 4, figs. 115, 116. 

 Nucula tenuis, Moller, Moll. Grrenl. 17. 

 Nucula ohliquata, Beck (1847), teste Mokch. 



