124 
MARINE SHELLS OF WEST COAST OF NORTH AMERICA 
Diplodonta aleutica Dali, 1901. 
Proc. U. S. N. M., 23:820; pi. 42, fig. 3. 
Shell large, coarse, chalky, with a papery dehiscent periostracum, 
usually with the exterior more or less eroded; form somewhat longer than 
high, tumid, equivalve and nearly equilateral, the anterior end of the shell 
less rotund than the posterior; surface in the adult with rather irregular 
and marked concentric lines of growth; ligament external, set in a groove, 
with well-marked nymphs; teeth normal, slender, and delicate, in the adult 
more or less defective; beaks low and inconspicuous, slightly nearer the 
anterior end; interior chalky, the margins entire, the posterior muscular 
impression larger than the anterior. The young are proportionately more 
elongate and less tumid, with smooth silky olivaceous periostracum. 
Longitude of adult, 26.5; altitude, 22; diameter, 14 mm. (Dali.) 
Type in U. S. N. M., No. 207325. Type locality, Kyska Harbor, 
Aleutians, in 8 fathoms. 
Range. Cape Lisburne, Arctic Ocean, to Aleutian Islands and Sitka 
Bay, Alaska. 
Diplodonta orbella Gould, 1852. 
Plate 6, figs. 5, 6. 
Proc. Bost. Soc . Nat. Hist., 4:90. Bost. Jour. Nat. Hist., pi. 15, fig. 3. 
Shell small, rather thin, subglobose, dingy white, marked with delicate 
lines of growth, which at some parts are more conspicuous than at others, 
and render the surface somewhat irregular; beaks very nearly median, not 
prominent; no distinct lunule in front of them; ligament prominent; 
extremities a little above the middle of altitude, very nearly symmetrical. 
Interior white, hinge with two direct teeth in the right valve, of which 
the anterior is smallest, and the posterior is bifid; and two in the left 
valve, of which the anterior is bifid and the posterior very oblique; lateral 
teeth none; muscular impressions faint, very large; pallial impression 
indistinct, composed of a series of polished dots. Length, %; height, %; 
diameter, % of an inch. (Gould.) 
Type locality, San Diego, California. 
Range. Pribiloff Islands to the Gulf of California. 
Diplodonta subquadrata Carpenter, 1855. 
Proc. Zool. Soc., p. 230. 
Testa subquadrata, valde insequilaterali, antice brevi; tenui, albo-flave- 
scente, epidermide, tenuissima; striis incrementi exillimis, ligamento sub- 
externo; dentibus cardinalibus parvis; lateralibus antico in utraque valva 
acuto, postico subobsoleto; cicatricibus muscularibus, antica a cardine 
remota, elongata, intus crenulata; postica irregulariter pyriformi; linea 
pallii margini appropinquante. Long., 76; lat., 89; alt., 57 poll. (Car¬ 
penter.) 
