132 TERTIARY AND QUATERNARY PKCTENS OF CALIFORNIA. 



Peoten (Pi-AViiocTENiuM) ciKCULAKis Sowprby var. ^quisulc:atus Carpenter. 



PI. L, figs. 1, Iff, and 1/.; aluo text tig. 1, p. 45; fig. 2, p. 46. 



1863. PeHeH [reiUricosus var.?] irquisulcatus Carpenter, Brit. AB.sn. Kept., 1863, p. 645. 



1865. Peden (?var. ) wqitmilcatus Oariienter, Ann. and Mag. Nat. Hist., 3d ser., vol. 15, 1865, p. 179. 



1869. Pedni rmtriro.ma Gabl), Pal. Cal., vol. 2, 1869, p. 104. 



1888. Pectcn H<jninilcntns Carpenter, Kiister and Kobelt, Conch. -Cab., vol. 17, pt. 2, Spondylus- und 



Pecten, no. 249, p. 269, taf. 71, tigs. 1 and 2. 

 1888. Pedeii rentrioofnis Cooper, Seventh Ann. Rept. Cal. St. Min., 1888, ji. 258 (Quaternary record 



..nly). 



1892. Peclni .rquisulcatus C'arpenter, Keep, West Coast Shells, 1892, p. 166, fig. 139. 



1893. Peclen wqivmilcatus Carpenter, Williamson, Proc. U. S. Nat. Mus., vol. 15, for 1892, p. 193. 

 1898. Peclen (irnlricofnis Yar."!) .rqitli^nlcalus Carpenter, Dall, Trans. Wagner Free Inst. Sci., vol. 3, pt. 



4, 1898, p. 711. 

 1898. Pectcn {Plagioctenmm) reiitricuHits Dall, Tnms. Wagner Free Inst. Sci., vol. 3, pt. 4, 1898, p. 710 

 (in part). 



1902. Pectcn lequisulcalus Carpenter, AVillianjson, Hull. So. Cal. Acad. Sci., vol. 1, .Nu. 5, May, 1902, 



pp. 51-64, pis. 4-6. 



1903. Pectcn (Pl(igioct.(inum) rof/rico.vi'.s Arm ild, Mem. Cal. Acad. Sci., vol. 3, 1903, \>. 114, pi. 11, figs. 



3, 3«. 



1903. Pecten {Plagioctenium) neicsami Arnold, Mem. Cal. Acad. Sci., vol. 3, 1903, p. 113, pi. 11, figs. 1 



and \a (a young shell). 



1904. Peclen lequisulcdliis Carpenter, Kecji, \\'est Am. Shells, p. 39, fig. 18. 



Description. — Adult .sliell averaging- about T.'i inillinieters in altitude, much 

 longer than high, the anterior portion of the di.sk being o])liquely produced; .sides 

 somewhat concave above; shell rather thin; margins .serrate. Right valve with 20 

 to 22 moderateljf prominent, tlat-topped ribs separated by much narrower, flat- 

 bottomed interspace.s; lateral areas about as wide as two normal ribs; whole surface 

 of disk sculptured by tine, sharp, looped, incremental lines; hinge line about live- 

 eighths length of disk; ears equal in length; anterior ear sculptured with fine 

 concentric lines und 7 or 8 more or less prominent radials;' hyssal notch deep 

 and sharply defined; posterior ear rectangularly truncated, and sculptured by 

 concentric lines and obsolete radials. Left valve with narrower ribs, which have 

 more sloping sides, and more prominent lamelhv of growth in the interspaces; 

 inter.spaces correspondingly broader; ears similar to those of right valve, except 

 that the posterior is not sharply notched. Color, white to liglit reddish yellow 

 blotched with shades of reddish ln'own, tiie left valve being always perceptibly 

 darker colored than the right. 



D/'mensionji. — Alt. 72 mm.; long. SiS nun.; hinge line 51 nun.; diameter .W mm.; 

 umbonal angle 10(1"'. 



This variety is more or l(\ss variable in regard to its convexity, obliquity, 

 relative width and height of ribs, and coloration. In the voung specimens the right 

 valve is pei'ceptibly more convex tiian the left; the ribs are relatively much squarer 

 and more prominent, the interspaces correspondingly narrower and deeper, the 

 hinge line comparatively longer, and the umbonal angle appears to be less, than 

 in the adults. The tendency of the ribs to tiatten out and the concentric sculpture 

 to l)ecome more irregular and rough as the shell grows older is evident to a greater 

 or less degree in nearlj^ all of the specimens examined. 



