92 MOLLUSCA FROM THE GREAT OOLITE. 



compressed, elongated figure, and a hinge, the dentition of which differs materially from 

 that of the better known forms of Corbis. The shells, likewise, are rather thin, the margins 

 not toothed, and the posterior side is always the larger of the two. 



Tancredia truncata, Lycett. Tab. XIII, fig. 11. 



Tancredia truncata, Lycett. Ann. and Mag. Nat. Hist., 1850, pi. 11, f. 10. 



Testa subtrigond, ovato-cuneatd ; umbonibus posticis ; latere postico, brevi, truncato; antico 

 elongato, margine superiore ejusdem recto, oblique-declivi ; margine inferiore subrecto. 



Shell subtrigonal, or ovately wedge-shaped ; umbones posterior ; posterior side short, 

 truncated ; anterior side elongated, its superior margin straight, sloping obliquely down- 

 wards, the extremity rounded ; basal margin nearly straight. 



The short posterior side slopes suddenly downwards, it is bounded by an obscure 

 angle or ridge. 



Height, 6^ lines; length, 13 lines; diameter through both the valves, 5 lines. Its 

 position is the shelly beds of the Great Oolite, in which it is somewhat rare. 



Localities. Minchinhampton and Bisley Commons. 



Tancredia brevis. Tab. XIII, fig. 8. 



Testa parvd subtrigond ; umbonibus submedianis ; latere postico brevi, angulo producto ; 

 marginibus acuminatis, margine inferiore elliptico. 



Shell small, subtrigonal; umbones submesial, depressed; posterior side sloping obliquely, 

 and having a prominent angle, which passes obliquely from the umbo to the postero- 

 inferior border; margin of the valves pointed at both extremities, the inferior margin 

 curved elliptically. 



Compared with T. axiniformis this species is much more short and convex, and it 

 always forms a prominent angle upon the posterior side, posterior to which the surface is 

 flattened, or even slightly excavated, the extremities of the valves being pointed. In its 

 geological range it accompanies the two other species ; it is everywhere common, and 

 certain layers of soft shelly Oolite beneath the planking of Minchinhampton Common are 

 entirely covered with its valves ; undoubtedly it is the most abundant bivalve in the district. 



Length, 1\ lines ; height, 4| lines. 



The Tancredia donaciformis, Lycett, 'Ann. and Mag. Nat. Hist.,' 1850, vol. vi, pi. xi, 

 fig. 8, approximates so nearly to our species that it is necessary to discriminate between 

 the two forms. The T. donaciformis is more lengthened, the umbones are mesial, but the 

 anterior side is more attenuated, its marginal slope being slightly concave, and its extremity 

 more pointed, so that the posterior side appears to be larger than the other ; it occurs in 

 the shelly free stone of the Inferior Oolite, Leckhampton Hill, in an abundance rivalling 

 our Great Oolite species. 



Locality. The whole of the Minchinhampton district. 



