110 MOLLUSC A FROM THE GREAT OOLITE. 



Thracia, Leach. 



Cokimya, Agassiz. 



Shell subtrigoual, inequivalve, inequilateral, rather flattened ; cardinal area distinctly 

 marked, the hinge margin forming a sudden declivity posterior to the umbones ; the 

 area is separated from the sides by a carina more or less visible ; the left valve is always 

 smaller than the right, its umbo is flatter or less elevated; the surface has concentric 

 plications more or less prominent ; the substance of the test is extremely thin, more 

 especially in the smaller valve ; the valves do not gape, or but very slightly, and the hinge 

 is destitute of teeth. From Tellina it is distinguished by the absence of teeth, and by its 

 wanting the lateral flexion which distinguishes that genus. 



Thracia studeri. Ag. sp. 



Tellina incekta, Thurm, Roemer, Verst. Nordd. Ool., p. 121, tab. 8, fig. 7. 



_ _ Gold/. Petref., tab. 140, fig. 14. 



Cokimya studeri, Ag. Etud. Crit., p. 267, tab. 35. 



Testa subaquivahi obovatd, convexd-pland, antice convexd ; margine curvato ; latere 

 postico abrupte compresso ; umbonibus mediants incequalibus, compressis ; lateribus plicis 

 concentrieis irregtdaribus. 



Shell subequivalve obovate, moderately convex, anterior side convex, its margin 

 curved, posterior side compressed, attenuated, and separated from the other portion of the 

 shell by an obtuse angle (sometimes imperfectly defined). The umbones are mesial and 

 contiguous, but not prominent nor large, the margins of the valves are close fitting ; the 

 sides of the valves have numerous irregular concentric plications. 



This species is more elongated, and has the posterior side more produced than our other 

 Great Oolite species ; the Cornbrash specimens have considerable variety of figure, with 

 respect to their height and to the distinctness of the posterior angle, irrespective of acci- 

 dental compression. 



Geological position and localities. In England it occurs in the Cornbrash of Wilts, 

 and in the Great Oolite of Northamptonshire. M. Agassiz states that it is abundant in 

 the Portlaudian beds of Porrentroy, Jura. Goldfuss records it in the upper oolite of 

 Hanover. 



Thracia curtansata. Tab. XIII, fig. \a, b. 



Testa convcxo-pland, subfrigond, subcequilaterali, et hevigatd ; umbonibus submedianis, 

 incequalibus incurvis ; latere posiico abbreviato ; valvd sinistra subpland, umbone parvo ; 

 plicis concentrieis tenuissimis irregtdaribus. 



