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 à 
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 549.1 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. 
CUMINGHA Sowerpy. 
Cumingia mutica Sowznnr. 
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 t9 
relegate it to Mya as a subgenus. 
Sphenia subequalis Darr, n. ep. 
Shell small, inequilateral, compressed, bluntly rounded at both ends, covered 
by a yellowish periostracum ; beaks low, nearly median, being 3.25 mm. from the 
