138 TERTIARY AND QUATERNARY PECTENS OF CALIFORNIA. 



Pecten (Psecjuamusium) davidsoni Dall. 



PI. L, tigs 4 and 4«. 



1897. Pi'cten Daridsovi Dall, Xautilus, vol. 11, No. S, Dec, 1897, p. 86. 



1902. Pecten Darkhoni Dal), I'roc. V. S. Nat. Mus., vol. 24, 1902, p, .559, pi. 40, figs. 5, 6. 



1904. Perlen dandmn'i Dall. Krcj.. West .\in. .Shells, p. 41, figs. 21fi, lib. 



The orio'inal dt'scription is as follows: 



Descriptio)!. — "'Sholl small. snborl)iciilar, cc )nipres.sed, waxen white, the left 

 valve with 21 roumlcd rihs. siu'inouiited In* (when not worn off) continuous rows of 

 minute subglobular scales, the interspaces wider, flat, and perfectly smooth, ear.s 

 veiy small, the anterior with five or six imbricated radii; sculpture obsolete near 

 the umbones; right valve scidptured with faint concentric impressed lines over the 

 whole .surface, and distally w-ith numerous minute, ol)solete, tine, scaly riblets: 

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

 radii, a well-marked .sinus leaving an imbricated fascicle and no ctenolium. Interior 

 polished, the left valve fluted internally in harmony with the external ribs." 



DhneniiionH. — Alt. 14 nnn. ; long. 1-tmni.; diameter 3.5 mm. 



This little shell somewhat resembles P. alankensis externally, but lacks the 

 radiating lin\? internally, is smaller, and has faint radiating sculpture on the right 

 valve, which is wanting in /'. iilaxh:»fiin. 



This deep sea form is known only in the living fauna. The type (U. S. N. M., 

 107747) is from the Davidson Bank, Alaska, in 280 fathoius, green-mud bottom. 



Living. Davidson Rank. Alaska. 280 fathoms; north of Unalaska, in Bering Sea, 

 351 fathoms (U. S. Fish Commission). 



Pecten (Pseudamusium) randolphi Dall. 

 PI. XLVIII, figs. 2 and 'la. 



1897. Pecten Randolph! Dall, Nautilus, vol. 11, No. 8, Dee., 1897, p. 86. 



1902. Pecteu Randolph) Dall, I'roc. U. S. Nat. Mus., vol. 24, 1902, p. 5.59, pi. 40. tig. 2. 



1904. Pecten randolphi Dall, Keeji. West Am. ."Shells, p. 41. 



The original description is as follows: 



Description. — "Shell small, thin, glassy, luisculptured. except l)y minute ''Cainp- 

 tonectes' striation which covers both valves, and more or less obscure concentric 

 undulations which are most distinct on the right valve near the umbo, and in some 

 specimens altogether absent; hinge straight and short, anterior ears distinct, posterior 

 ears not defined by any fold or sinus, outline suborbicidar, valves compressed, espe- 

 cially the right one; right anterior eai- with six small imbricated radii above, below 

 a wide, transversely striated fascicle derived from a well-marked byssal sinus; cteno- 

 lium with four or five fiuictional spines." 



Dimensions. — Alt. 27.5 mm.: long. 2<') mm.; diameter 5 mm. 



This species is the living representative of the group consisting of the three ver\' 

 closely allied, if net identical forms. 1\ pecl'huini. P. jieilroanux, and /'. randolphi. 

 The poor state of preservation of the fossil specimens precludes any definite state- 



