BIVALVIA. 127 



The central ridge divides into several very elevated acute costae ; the interstitial spaces 

 of which, form acute angles with them ; the substance of the test is thick. The Ostrea 

 sulcifera, Phil., of which we have a specimen from Whitwell, Yorkshire, is only the germ 

 of this large species, in which the central longitudinal smooth ridge has not divided to form 

 the great posterior denticulate plications, the latter change having taken place at a subse- 

 quent period of its growth. In the adult condition the figure is sometimes a lengthened 

 oval or oblong as in 0. sulcifera, but, in other instances, which probably represent the 

 final stage, the lower and larger plications spread out laterally, giving the shell a fan_ 

 shaped contour. 



Geological position and localities. We have received 0. Marshii from the grey lime- 

 stone of the Great Oolite near Scarborough ; we have collected it in the Cornbrash near to 

 Malmesbury, and it occurs not uncommonly in the upper division of the Inferior Oolite in 

 the Cotteswolds. Ostrea sulcifera is from the Great Oolite of Whitwell, Yorkshire. 



GRTPH^A MIMA, Phil. Tab. XIV, fig. 5. 



GRYPH^EA MIMA, Phil. Geol. York., 1, t. 4, f. 6. 



Testa parvd obliqud, subglobosd, valvd convexd, rugis concentricis magnis ; area adherenti 

 magnd, alterd convcxo-pland. 



Shell small, oblique, subglobose, the larger valve convex, rugose, with large concentric 

 folds ; the adherent surface subterminal, large ; the smaller valve more smooth, slightly 

 convex. 



More globose than Ostrea rugosa, Sow., and destitute of the marginal plications ; in 

 other respects it much resembles that little species. 



Height, 6 lines ; lateral diameter, 5 lines ; diameter through both the valves, 3 lines. 



PECTEN DEMISSUS, Phil. Tab. XIV, fig. 7. 



PECTEN DEMISSUS, Phil. Geol. York., 1, t. 6, f. 5. 

 Goldf. Pet., p. 74, t. 99, f. 2. 



Testa suborbiculari planatd; umbonibus parvis acutis; auriculis parvis aquaUbm, valvd 

 dextrd subpland, valvd sinistrd convexio; lateribus cequalibus marginibus rotundis; superficii 

 glabro lineis tenuissimis concentris, aliis subobsoletis radiantibw decussatis. 



Shell suborbicular, depressed, smooth and shining ; umbones small, acute ; auricles 

 small, equal, rising slightly at their extremities, their outer borders curving obliquely 

 downwards ; the margin of the valves slope downwards from the umbones nearly at an 

 equal angle on each side, (about 40 to the axis of the valves) and the margins and base 

 are regularly rounded ; the right valve has only a very slight convexity, and sometimes is 

 traversed on each side obliquely by a slight furrow diverging from the umbo ; the left 



