1915] 



Clark: Fauna of the San Pablo Group 



447 



P. alamedensis Yates is a very common species in the Upper San 

 Pablo Group, of which it appears to be characteristic. It is notice- 

 able for its large size and the thickness of the prismatic outer layer. 

 Some of the specimens collected have a maximum antero-posterior 

 length of ten inches, with a maximum width at right angles to that of 

 about five and one-half inches. 



OSTREA BOURGEOISII Remond 

 Plate 43 



Ostrea bourgeoisii Remond. Proc. Calif. Acad. Sci., vol. 3, p. 13, 1863. 

 Ostrea bourgeoisii Remond. Palaeontology of California, vol. 2, p. 36, pi. 

 11, fig. 57, 57a, 1863. 

 The description is from two lower valves, from the collections of Mr. 1 'Abbe 

 Bourgeois (Pont-Levoy, Prance), and Mr. Pioche (San Francisco). 



Shell sub-oval, higher than long, strongly contracted near the cardinal area, 

 sub-rounded on the ventral margin; inferior valve comparatively thin, convex, 

 irregular exteriorly, with remote, somewhat rugose, plaits of growth. Liga- 

 ment fosset long, profound, minutely wrinkled and finely striated, oblique and 

 turned downwards; muscular impression very large, oblique, and sub-central, 

 somewhat prominent. 



Locality: vicinity of Kirker's Pass, from a late Tertiary bed. 



0. bourgeoisii Remond has been confused in the past with 0. titan 

 Conrad. The writer in a previous paper (Univ. Calif. Publ. Bull. 

 Dept. G-eol., vol. 7, no. 4. p. 54, 1912) listed it as 0. titan Conrad. 

 Other writers previous to this made the same mistake. Gabb reports 

 0. titan Conrad from Kirker Pass. All the specimens that the writer 

 has seen from that locality, including specimens of the old State 

 Survey in the collections of the University of California and labeled 

 0. titan Conrad, belong to the species 0. bourgeoisii Remond. So far 

 the writer has not seen a specimen of 0. titan Conrad from the Lower 

 San Pablo group or the upper part of the Monterey group. In the 

 Lower Monterey there is an oyster that is very close to, if not identical 

 with, 0. titan Conrad as seen in the Santa Margarita beds of San Luis 

 Obispo County. 



0. bourgeoisii Remond somewhat resembles 0. titan Conrad in its 

 size and thickness, though it is never as large. The maximum length 

 as measured from the beak through the muscle impression is about 

 eight to ten inches, with a maximum width at right angles to this of 

 about five inches. Both valves vary considerably in thickness ; some- 

 times the shell of the ventral valve has a thickness of from two to three 

 inches. The ventral valve is usually only slightly more convex than 



