86 TERTIARY AND QUATERNARY PKCTENS OF CALIFORNIA. 



Peoten (Placioctenium) discus Connid. 



PI. XXVII, tigs, I, 1», 1", ;i, and 4. 



1857. Pecten iKscuil Conrad, Pac. R. R. Eept., vol. 7, pt. 2, 1857, p. 190, pi. X, fig. 1. 



1869. Pecten disms Conrad, Gabh, Pal. Cal., vol. 2, 1869, p. 122. 



1888. Pecten discus Conrad, Cooper, Seventh Ann. Kept. Cal. St. Min., 1888, p. 257. 



1888. Peclen pahloeims Conrad, Cooper, Seventh Ann. Kept. Cal. St. Min., 1888, p. 258 (in part). 



1894. Not Pecten (//sow Cooper, Bull. Cal. St.^Min. Bureau, No. 4, pt. 5, 1894, p. 57, pi. 5, figs. 55, 56. 



{= P. andersoni n. sp. ) 



1898. Pecten {<:hlr(miiK'>) rf/.fou.s- Conrad, Dall, Trant.. Wagner Free Inst. Sci., vol. 3, pt. 4, 1898, i>. 704. 



Di'itcription. — Shell averaging about 50 iiiillimeter.'s in altitude, about as long- 

 a.s high, cquivalve, ecjuilateral, thin, rather compressed, and with smooth margins; 

 umbonal angle large, averaging about 108-; sides .somewhat concave above. Right 

 valve with IT to 2(l moderately strong, rounded ril)s, separated by rather flat 

 interspaces about equal in width to the ribs; surface sculptured by regular, 

 elevated, wavy, incremental lines and, at irregular intervals, generally liy more 

 or less prominent lines of interrupted growtii; hinge line about one-half length 

 of disk; ears unequal, the antcrit)r showing several strong radials, while on the 

 posterior the radials are obsolete; both ears sculptured by fine concentric lines. 

 Left valve similar to right; ears more or less distinctly radially ridged and 

 obliquely truncated. 



Diiiirnsions. — Alt. 48 mm.; long. 50 mm.; hinge line 24 nun.; diameter 8 mm.; 

 umbonal angle 108". 



This species, of which the type is a poor specimen, l)cing preserved as a cast in 

 indurated clay, is characterized by its moderately strong, convex rib.s, compressed 

 disk, and strikingly unecj[ual ears on the right valve. Specimens of P. dtscux having 

 the shell material preserved always show moderately high, convex rii)s; the internal 

 casts (in which form the species is genei'ally found) have the ribs more or less 

 squarish, and low. The specimens having the fewc^st niunbcr of ribs have the widest 

 lateral areas or submargins. The type does not show the radial sculpture found in 

 most cases on the ears of the left valve. 



P. discm is closely allied to P. ijahloennix. l)ut has the constant difference of 

 larger size when adult and stronger rounded ribs instead of weak squarish ones. It 

 occurs in approximately the same horizon as /*. piiMoenx'ix and seems to represent the 

 latter in what is proliably the equivalent of the San Pablo (upper Miocene) forma- 

 tion in the region south of the typical San Pablo Bay locality, although at some 

 localities it has Ijeen found in beds of probable middle Miocene age. 



About the head of Crescent Creek, near Oil City, Fresno County, P. discus is 

 associated with the following; Ocinehra near canveUltia, Trochita Jilosa, Mytilus 

 viiitheii'Sdnii (*), Arra sp. indet., and Valuta (?) sp. This appears to be an uncertain 

 horizon in the Miocene. 



RANGE. 



Miocene (upper). Walnut Creek, Contra Costa County (Univ. of Cal. collection); 

 Santa Margarita, San Luis Obispo County (J. H. Henry); Assuncion ranch, San 

 Luis Obispo County (Fairbanks; Arnold; R. Moran); Huasua ranch, 20 miles, 

 north of Santa Maria, Santa Barbara County (C. E. Knecht). 



