448 



University of California Publications in Geology [Vol. 8 



the upper valve but is usually somewhat thicker. The hue radiating 

 ridges on the surface, described by Remond, are not constant, ap- 

 pearing on only a few specimens that the writer has seen. 



0. bourgeoisii Remond differs from 0. titan Conrad in that the 

 ligamental pit is narrower and the ligamental callus is heavier ; on 

 Ostrea titan Conrad the ligamental pit extends nearly across the an- 

 terior end of the shell ; the muscle impression on 0. bourgeoisii Remond 

 is much smaller and is nearer the posterior end of the shell than on 

 0. titan Conrad; on the latter species the muscle impression is some- 

 times nearly half the width of the shell; the ventral valve of 0. titan 

 Conrad is usually more strongly convex. A large number of specimens 

 of both species were examined and these differences were found to be 

 constant. 



Occurrence. — Very common in the Lower San Pablo group at 

 Kirker Pass, University of California locality 2140 ; also from the 

 south side of Mount Diablo in the Lower San Pablo Group and Upper 

 Monterey Group. 



PECTEN (PLAGIOCTENIUM) BLLINEATUS, n. sp. 

 Plate 64, figures 3 and 4 



Shell thin, medium in size, equivalved and equilateral, usually 

 longer than high. Dorsal edges gently concave, the anterior dorsal 

 edge usually being shorter than the posterior ; ventral edge quite 

 strongly and regularly arcuate. Apical angle averaging about 102°. 

 Hinge plate about half as long as the length of the shell. Both valves 

 gently convex and similarly sculptured by 18 to 20 low, flat-topped, 

 radiating ribs with interspaces little wider than the tops of the ribs. 

 In each interspace and almost filling it there is a low, broad, flat- 

 topped interrib. On some specimens these broad interribs become 

 almost as prominent as the ribs themselves ; this is more especially 

 noticeable near the dorsal edges, where the ribbing becomes finer. 

 Anterior ear of right valve notched, bearing three to four fine, radi- 

 ating riblets ; posterior edge of posterior ear slightly oblique, bearing 

 very faint, almost obsolete, radiating riblets. Ears of left valve similar 

 to those of the right valve, except that the anterior ear is not notched. 



P. bilineatus, n. sp., has been confused with P. raymondi, n. sp., 

 and P. pabloensis Conrad, which species also occur in the same locality. 

 Arnold's description of P. pabloensis Conrad (U. S. G. S. Prof. Paper, 

 No. 47, pp. 87-89) in part applies very well to P. bilineatus rather 

 than to what is taken by the writer as being the true P. pabloensis Con- 



