NO. 1139. DEEP-WATER MOLLUSC A VERRILL AND BUSH. 





Family POEOMYIDyE. 



POROMYA SUBLEVIS Verrill, variety MICRODONTA Dall. 

 (Plates LXXVI, figs. 1, 2; LXXXVII. fig. 1.) 



Poromya suMeris VKKRILL, Trans. Conn. Acad., VI, pp. 221, 277, pi. xxxn, fig. 

 21, 1884; Expl. Albatross, Report U. S. Coin. Fish and Fisheries for 1883, 

 p. 574, pi. xxx, fig. 128, 1885. BALL, Hull. Mus. Corup. Zool., XII, pp. 281, 

 282, 1886; XVIII, p. 448, 1889 (variety?); Bull. U. S. Nat. Mus., No. 37, p. 

 68, pi. LXV, tig. 128, 1889. 



Poromi/d mh-rodonta BALL, Proc. U. S. Nat. Mus., XII, p. 290, pi. vin, fig. (5, 1889 

 (variety?). 



Shell rather large, thick, well-rounded, cordate, iuequivalved, very 

 tumid, with very large, prominent umbos which are strongly curved 

 forward spirally; beaks large; lunule small, cordate, often not very 

 distinct. The shell varies considerably in outline and size and eleva- 

 tion of the umbos; in most specimens the height equals or slightly 

 exceeds the length; the outline of the cavity of the shell is usually 

 somewhat elliptical, the length decidedly exceeding the height, but 

 sometimes it is nearly circular. The anterior and posterior margins are 

 usually pretty evenly rounded; the ventral margin usually projects a 

 little in the middle; the beak is situated in front of the median line. 

 Externally the shell is nearly smooth and is covered with a thin, 

 closely adherent, brownish-yellow epidermis; under a lens the sur- 

 face shows minute raised points or granules which are arranged in 

 radial rows that become more distinct and crowded posteriorly but 

 for the most part disappear on the most prominent part of the umbos. 

 These granule-like points are variable in number and distinctness, in 

 some specimens being nearly obsolete and in others distinct and regu- 

 larly arranged; the epidermis often also shows fine lines of growth; 

 the beaks are smooth and shining. The left valve has a posterior, 

 wave-like, radial depression, and behind this a low, rounded ridge 

 projecting at the margin as a slight siphonal lobe; in the right valve, 

 the corresponding lobe and depression are only faintly marked in most 

 cases. The right valve is larger than the left and overlaps it consider- 

 ably along the ventral margin and both in front of and behind the 

 beaks. The interior is pearly and often shows radial striatious. The 

 hinge-margin is considerably thickened and strongly curved; the right 

 valve has a large, thick, somewhat rounded tooth just beneath the 

 beak and aduate to the inner surface of the shell, for some distance 

 within the cavity of the beak and to the thickened edge behind the 

 beak, but separated from the anterior margin by a deep, curved furrow 

 in the lunular area; the lunular margin is convex and somewhat everted, 

 separated from the rest of the anterior margin by a slight notch. The 

 ligament is rather long and well-rounded and its groove extends for- 

 ward in a curved furrow under the beak; it extends backward in a 

 curved line parallel with the margin of the shell for some distance 





