PLEISTOCENE PECTENS. 133 



The Pleistoceii(> shells, as a rule, are laoi-e convex (the right laln^ noticeably 

 more so than the left), somewhat less oblique, and the ribs are slightly bi-oader and 

 more compact, than the living specimens. The young of /'. var. xqui<<i(Jefifiif< have 

 the characteristics of the Pleistocene form and are scarcely distinguishable from the 

 yoimg of P. cerro.ifiix/'s. 



P. var. a'fjui.ndciitiis varies from the typical /'. circnlariK of the (iulf of Cali- 

 fornia and west Mexican coast fauna by its larger size when adult, flatter and thiiuier 

 disk, iiai'rower ribs, and more sulidued coloi'ation. These two forms seem to have 

 separated during the Pleistocene, the living shells not appearing to give a satisfac- 

 tory connection, while a large series of Pleistocene specimens shows an obvious 

 gradation. Doctor Dall (1898. p. (JO-t). in speaking of the influence of environment 

 upon the Pectens, thus mentions th(> relation between P. <yrciil(iri>< and /'. var. 

 cequisulcafiiK: "As in nianuiials and l>irds, the same species in the northern part of its 

 [Pecten] range is larger than in the south, unless it is a distinctively tropical species. 

 But in color the rule is reversed, the southern specimens being lighter and more 

 brightly tinted than the northern ones in the same species. The specimens which 

 live in deep water and swim actively are usually thinner shelled and smoother, 

 while those which inhabit the lagoons are heavier, have more conspicuous concenti'ic 

 sculpture and more solid shells. These diflerences ai-e very marked in our common 

 east coast P. irrad/'atix, of which /'. ili.\Iiie((tus Say is the southern lagoon form; and 

 parallel diflerences appear in the similarly related /'. rentr/'rosv.s [= P. c/rni/nrt's] and 

 its variety mqimidcatus on the Pacific coast * * *." 



/'. var. xquistilcafus ranges from the Pleistocene to the Recent fauna. 



KA.NGE. 



Living. Santa Barbara to Pacific side of Lower California. 



Pleistocene. San Pedro; San Diego; Lower California (Hemphill. Stearns, Orcutt); 

 Ventura; San Pedro; San Diego (Arnold); Springville, Ventura County (Bowers). 



Pecten (Propeamusium) alaskensis Dall. 

 PI. LlII, figs. 2, 2a, and 3. 



1872. Pecten (PsendrtmnMnm?) Alaskeiisis Dall, Am. Jour. Conch., vol. 7, 1S72, p. 1.5-5, pi. 1(1, tig'. 4. 



1886. Pecten {Propeamitsiiun) aliiskensis Dall, Bull. Mas. Comp. Zool., vol. 12, no. (i, 1S88, p. 21.5, jil. 



5, figs. 7, 7a. 



1887. Not Pecten tilankensix Whiteaves, Trans. Roy. Soc. Canada, vol.4, set'. 4, 1887. p. 119 (=P. 



mncourerensls Whiteaves, 1893). 



1888. Pecten alaskensis Dall, Kiister and Kobelt, t!onch.-Cab., vol. 17, i)t. 2, Spondyliis unci Peoten 



sp. 220, p. 245, taf. 64, figs. 7, 8. 



1889. Pecten {Propeamusium) alaskensis DaU, Bull. I'. 8. Nat. Miis., No. 37, 18SH, pi. .5, tig..;. 7, 7,(. 

 1893. Pecten Alaskensis Dall, Newcombe, Rejit. on Mar. Sh. Brit. Columbia, Bull. Nat. Hist. Soc. 



Brit. Columbia, 1893, art. 5, p. 53. 

 1895. Pecten (Pseudamiisimn) Alaskensis Dall, Taylor, Trans. Rny. Soc. Canada, 2d ser., vol. 1, 1895 

 sec. 4, p. 27. 



1897. Amasiam (Propeamusium) Alaskensis DaXX, Verrill, Trans. Conn. .\cad. Sci., vol. 10, p. 65. 



1898. Peclen (Propeamusium) alaskensis Dall, Trans. Wagner Free Inst. Sci., vol. 3, pt. 4, 1898, p. 711. 



The oi'iginal description is as follows: 



Descripthm. — "Shell nearly equilateral, inequivalve, flesh color, with a lilush of 

 salmon color on the mnbo of the superior valve. Internally white, the salmon color 



