146 PROCEEDINGS OF THE NATIONAL MUSEUM vol. i09 



438 feet; 9 (from 7 to 17 mm.) from 453 feet; and 5 (from 6 to 15 

 mm.) from 522 feet. 



Other material examined: Over 100 specimens from localities 

 ranging from the Sea Horse Islands through Bering Strait, Plover 

 Bay, the Shumagins, the Aleutians, and Juneau, Alaska. 



Discussion: The ^'alhus" refers to the white of the inside of the 

 valves. Externally, the valves, as well as the girdle, range between 

 a dusty white or cream color (pi. 17, tig. 4) to a condition in which 

 brown predominates (pi. 17, fig. 3), the dark color being due to 

 microscopic specks of pigment. As in specimens from other locahties, 

 the darker colored ones predominate at Point Barrow, but in several 

 specimens from 420 feet the white area is greater than that with 

 brown pigmentation. 



A single specimen (USNM 214438) from San Francisco Bay labeled 

 T. alhus does not belong to this species. The "spicules" of the girdle 

 are too fine for T. alhus, there is a nodule on the posterior valve that 

 is not present in T. alhus, and the apex of the valves is drawn into a 

 more prominent point than in T. alhus. The lateral areas are ill de- 

 fined. A small specimen (USNM 218739) from Monterey Bay is also 

 incorrectly named T. alhus — the plates are narrower, the sculpture is 

 coarser and in definite rows rather than in an over-aU effect as in T. 

 alhus, and, as in the specimen from San Francisco, it also has the 

 nodule on the posterior valve. In stiU another specimen (USNM 

 159351), from San Diego, the "spicules" of the girdle are fine, giving 

 an impression of pihng, the sculpture is coarser than in T. alhus, and 

 the nodule is present. Time did not permit the identification of 

 these 3 specimens, which may represent a new species. 



However, 9 specimens (USNM 224045) from the Stearns Collec- 

 tion, labeled "West Coast of Lower Cahfornia," are T. alhus. 



Distribution: Point Barrow south and east to Juneau, Alaska, 

 and, if the above localit}^ record is correct, the west coast of Lpwer 

 Cahfornia; the Gulf of St. Lawi'ence south to Massachusetts Bay; 

 the British Isles; Spitzbergen; Norway. 



