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 Qoniomya americana, 

 described by Dr. Hayden and myself from the Upper Cretaceous rocks 

 of the Upper Missouri (see Proc. Acad. Nat. Sci. Philad., viii, 81), 

 but it is proportionally deeper from the beaks to the base, and differs in 

 having the costse 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 THEACIA, Leacli. 

 Thracia? occrDENTALis,*Meek. 



Plate 2, figs. 3 and 3 a. 

 Thracia ? ocddentalis, Mewk (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 um- 

 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 provisionally that they have been placed in the genus Thracia. 



Locality and position. — Nanaimo, Vancouver's Island 5 Cretaceous. 



Thracia ? subtruncata, Meek. 



Plate 2, figs. 4 and 4 a. 



Thracia ? sicbtruncata, Meek (1857), Trans. Albany Institute, iv, 44. 



Shell transversely subelliptical or subovate, thin and moderately com- 

 pressed, 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 be- 

 hind them ; beaks subcentral, small, compressed, and but moderately 

 prominent. Surface (of worn specimens) showing traces of rather dis- 

 tinct 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 



