CRETACEOUS FOSSILS. '203 



OSTREA, Linn. 

 O. Idriaensis, n. s. 



PI. 33, Fig. 103, b, <■, d. 

 PI. 34, Fig. 103, 103 a. 



Shell moderately large, oblique, often curved, heavy; lower 

 valve usually deep, more oblique than the upper; hinge straight 

 or deflected to the left, median groove pretty strongly marked; 

 internal margins in the young shell, finely crenulated or pitted 

 towards the beak, this character disappearing to a great extent in 

 the adult. Surface of both valves roughly and very irregularly 

 squamose. Muscular scar reuiform to semicircular, and placed 

 distinctly to one side of the middle. 



Length, from three to six inches. The figures are nil natural size, the greatot 

 thickness of the specimen, fig. 103, is 1.25 inch, the fragment, 103 n, has about an 

 equal thickness, indicating a shell when entire, perhaps three inches through both 

 valves. 



From the Tejon Group, on the same horizon as the beds at Marsh's, near Mount 

 Diablo. Locality, about two miles east of the Hacienda at New Idria, where it is 

 very abundant. 



The obliquity of the hinge in this shell is quite a variable character. It is well 

 represented by the figures 103 and 103 d, in its greatest extent ; figure 103 a is a 

 transition form, while a specimen of an upper valve before me, has the hinge 

 nearly straight. 



O. APPRESSA, 11. S. 

 PI. 34, Fig. 104, 104 a. 



Shell thin, flat, more or less equilateral, valves nearly equal, 

 usually about two-fifths longer thau wide. Surface covered by 

 numerous, thin, squamose plates. Hinge flat, large; margins 

 simple, sometimes subsquamose. Muscular scar small, oblique. 



Figures, natural size. Some specimens were found seven inches long. 

 I found this shell forming a stratum several feet thick, adjoining a bed of coal, 

 and in associated strata were familiar species characteristic of the Tejon Group. 



