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Show 363 curves of its lines of growth, however, give a tolerably correct idea of its outline, and show that its posterior side is rather distinctly truncated. This end also Seems to have been gaping, while the other extremity has the appearance of being closed. The substance of the shell is evidently * very thin; and it is probable unworn specimens will show the surface to be granular, as usual the case in this and allied genera. In its general appearance, this species resembles Ooniamya americana, described by Dr. Hayden and myself from the Upper Cretaceous rocks of the Upper Missouri ( see Proc. Acad. Nat. ScL Philad., viii, 81), but it is proportionally deeper from the beaks to the base, and differs in having the costae on the posterior side of its valves curving forward, instead of being directed obliquely backward. Other differences will probably be seen on comparing better specimens. Locality and position.- Nanaimo, Vancouver's Island; Cretaceous. Genus THKACIA, Leach. THBACIA! OCCIDENTALISM Meek. Plate 2, figs. 3 and 3 a. Thraoia t occidental^, Meek ( 1857), Trans. Albany Inst., iv, 43. Shell elliptical, moderately compressed, thin and fragile; right valve apparently a little more convex than the other; extremities rather narrowly rounded; outline of base forming a broad [ semi- elliptic curve; dorsal margin sloping a little in front of the beaks, and less distinctly behind them; beaks nearly central, small, compressed, and not much elevated; postero- dorsal region compressed behind the posterior urn-bonal slopes. Surface ( of worn specimens) showing obscure wrinkles of growth. Length about 2.08 inches; height, 1.25 inch; breadth, 0.52 inch. As none of the specimens of this and the following species are in a condition to show the hinge, pallial line, or muscular impressions, it is only provisionaUy « that they have been placed in the genus Thracia. Locality and position.- Nanaimo, Vancouver's Island; Cretaceous. THRACIA ? SUBTEUNCATA, Meek. Plate 2, figs. 4 and 4 a. Thracia t subtruncata, Meek ( 1857), Trans. Albany Institute, iv, 44. Shell transversely subelliptical or subovate, thin and moderately compressed, the right valve being a little more convex than the other; anterior side rather narrowly rounded; posterior extremity subtrun-cated; basal margin forming nearly an elliptic curve, but rounding up more gradually in front than behind; cardinal margin sloping with a slightly convex outline in front of the beaks, and nearly horizontal behind them; beaks subcentral, small, compressed, and but moderately prominent. Surface ( of worn specimens) showing traces of rather distinct lines of growth. From immediately behind the beaks, there is ( in internal casts) a distinct depression passing obliquely backward and downward just back of the posterior umbonal slope of each valve. Hinge and other internal characters unknown. Length, 1.84 inches; height, 1.14 inches; breadth about 0.50 inch. It is possible that this may not be distinct from the last, but, judging from the specimens yet seen, I am inclined to think these two forms belong to different species. That now under consideration differs from |
