20 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 23 



internal lirae of the right valve much less prominent next to the sub- 

 margins and almost obsolete over the center of the disk; the exterior of 

 the left valve very seldom has any "pseudo-ribs," only faint ridges with 

 very fine concentric lamellae near the ventral margin. 



Geographical range: Bering Sea; Aleutian Islands to Magdalena 

 Bay, western Lower California; northern Japan and Korea. 



G eochronological range: Pliocene, Pleistocene, Recent. 



Bathyrnetric range: Recorded in 10 to 200 fathoms, possibly living 

 deeper. 



Ecological data: None available. 



Propeamussium (Parvamussium) davidsoni (Dall) 1897 



Plate 4 



Pecten Davidsoni Dall, 1897, p. 86. "On the Davidson Bank, Alaska, 

 in 280 fathoms, green mud, and north of Unalashka, in Bering 

 Sea, in 351 fathoms, sand."— Dall, 1902, p. 559, pi. 40, figs. 5, 6. 

 [Same localities.] 

 Pecten (Pseudamusiurn) davidsoni Dall. Arnold, 1906, p. 138, pi. 50, 

 figs. 4, 4a. [Same localities as Dall.] — Dall, 1921, p. 20. David- 

 son and Bowers Banks, Bering Sea. — Oldroyd, 1924a, p. 61, pi. 14, 

 figs. 5, 6. [Figures erroneously referred to P. randolphi Dall under 

 that species in text (p. 59) and on plate index opposite plate 14.] 

 Pecten (Propeamussium) davidsoni Dall. Grant & Gale, 1931, p. 234. 



Alaska. 

 Pecten (Pseudamussium) davidsoni Dall. La Rocque, 1953, p. 34. 

 Davidson and Bowers Banks, Bering Sea. Alaska. 

 Holotype: U.S. National Museum. 



Type locality : Davidson Bank (Bering Sea), Alaska, in 280 fathoms. 

 Original description : Shell small, suborbicular, compressed, waxen 

 white, the left valve with 21 rounded ribs, surmounted by (when not 

 worn off) continuous rows of minute subglobular scales, the interspaces 

 wider, flat and perfectly smooth, ears very small, the anterior with five 

 or six imbricated radii ; sculpture obsolete near the umbones ; right valve 

 sculptured with faint concentric impressed lines over the whole surface, 

 and distally with numerous minute, obsolete, fine, scaly riblets ; posterior 

 ear transversely striated, very small, anterior one with four or five scaly 

 radii, a well-marked sinus leaving an imbricated fasciole and no ctenoli- 

 um. Interior polished, the left valve fluted in harmony with the external 

 ribs. Alt. 14, lat. 14, diam. 3.5 mm. 



1 



