422 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
the slightly arcuate base ; posterior dorsal margin straight, more rapidly descend- 
ing, the posterior end with a narrow, oblique truncation, slightly angular below ; 
surface sculptured with close, minute, concentric lines giving it a silky lustre, 
and with occasional stronger sulci, indicating resting stages; above the posterior 
angle the lines are coarser and more elevated, and near the margins are traces of a 
thin, brownish periostracum ; on the disk are also traces of opaque, fine radiating 
lines, but these do not seem to be reflected in the sculpture ; hinge line narrow, 
within the left valve three small cardinals, the anterior pair closely adjacent, the 
other more distant; inner surface polished, pallial sinus large, ovate, horizontal, 
extending in front of the middle of the shell, only slightly coincident with the 
pallial line below. Lon. of shell, 34; of posterior end, 13; alt. 22; diam. of left 
valve, 5 mm. 
U.S. S. “Albatross,” station 2355, Gulf of Panama, in 182 fathoms, mud, 
bottom temperature 54°.1 F. U.S. N. Mus. 122,936. 
Only a single left valve was obtained. The most nearly related species is 
M. (Psammotreta) aurora Hanley, which has a different hinge, and is a broader 
shell. 
Semelidae. 
CUMUEUNGEA Sowerby. 
Cumingia mutica Sowersy. 
At Tomé, Chile, from the mud upon the anchor, were obtained some fragments 
and small specimens of a Cumingia which are probably the young of C. mutica. 
(AsTHENODONTA) 
Myacea. 
Myacidae. 
SPHENIA Turton. 
The distinction between this genus and Mya is extremely slight. It has pro- 
portionately shorter siphons, and the species recognized are all of small size, but 
it seems impracticable to discover other differences and I am strongly tempted to 
relegate it to Mya as a subgenus. 
Sphenia subequalis Datt, n. sp. 
Shell small, inequilateral, compressed, bluntly rounded at both ends, covered 
by a yellowish periostracum ; beaks low, nearly median, being 3.25 mm. from the 
