820 PROCEEDINGS OF THE NATIONAL MUSEUM. voL.xxmi 



subumbonal tooth of the right valve is prominent and strong, the 

 inflected tooth-like process of the left valve is well developed. Mar- 

 gins of the valves smooth, interior polished, with some obscure radial 

 striae; muscular and pallial impressions normal. In the animal the 

 hepatic glands project in an arborescent manner from each side of the 

 comparatively' insigniticant bodymass, the gills are normal and rather 

 small. Alt. of shell 6.0, Ion. 6.2, diam. 3.3 mm. The specimen fig- 

 ured is from Iliuliuk, Alaska, in 19 fathoms, mud. 



Ranges over the North Pacific region from Ik^ring Strait to North-] 

 ern Japan on the west and Catalina Island, California, on the south 

 and east, in 5 to 167 fathoms, muddy or sandy bottom. 



I have described this shell with some hesitation, as it may prove to 

 be the normally rotund form of which A. sericata Carpenter is an 

 ol)lique and ovate variety, but until this is shown it would seem as if 

 the differences are worthy of systematic recognition. The Carpen- 

 terian type measures in alt. -1.5, Ion. 4.0, and diam. 2.6 mm.; the 

 beaks are higher and more recurved, the periostracum pale yellowish 

 gray and papery. 



DIPLODONTA (TORELLI Jeffreys, var.?) ALEUTICA, new species. 

 (PlateXLII,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, cquivalve 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 proportionall}^ more elongate and less 

 tumid, with a smooth silky olivaceous periostracum. Lon. of adult, 

 26.5, alt. 22.0, diam. 14.0 mm. ; of young shell (figured), lon. 15, alt. 12, 

 diam. 6 mm. The figured specimen is from 10 fathoms, sandy mud, 

 in Kyska Harbor, Aleutians; the t\'pe from 8 fathoms in the same 

 body of water. 



The species ranges from the Pribilof Islands to the Aleutian chain 

 and eastward to the Shumagin Islands. It has not been found in the 

 dredgings north of the Pribilof group. 



The adult shell looks remarkably like the D. torelli Jeffreys, of the 

 North Atlantic and SpitsbergcMi seas. Owing to their usual state of 

 erosion it is difficult to compare adults, but the young of D. toreUi 

 has a coarsely wrinkled, yellowish periostracum and a rougher surface 

 than that of aleutica. The ranges of the two are separated by an 

 immense distance, but, whether due to analogous environment or con- 

 genetic origin, the adult shells are nearly indistinguishable. 



