BIVALVIA. 93 



Tancredia curtansata, Phil., sp. Tab. XIII, fig. la, b. 



Corbula curtansata, Phil. Geol. York., 1, t. 3, f. 27. 



Testa ovato-elongatd ; umbonibus medianis, parvis ; antice compressd, acuminata, postice 

 convexd ; margine antico oblique declivi concavo ; basi ellipticd curvatd. 



Shell ovately elongated ; umbones small, mesial ; anterior side compressed, its 

 extremity pointed ; posterior side moderately convex, its margin slightly rounded ; antero- 

 superior border obliquely sloping and concave ; base elliptically curved. 



This is the largest of the Great Oolite species of this genus, it is moderately abundant 

 in the shelly beds ; specimens vary much in the convexity of the valves. 



Height, 10 lines; length, 15 lines; diameter through the valves, 7 lines. 



Localities. Minchinhampton, in the Great Oolite ; Malton, in the Coralline Oolite. 



Tancredia axiniformis, Phil., sp. Tab. XIII, fig. 6a, b. 



Nucula axiniformis, Phillips. Geol. York., 1, t.' 11, f. 13. 



Tancredia Extensa, Lycett. Ann. and Mag. Nat. Hist., 1850, pi. 2, f. 9. 



— axiniformis, Morris. Journ. Geol. Soc., 1853, v. 9, p. 341, pi. 14, f. 4. 



Testa ovato-trigond elongatd, convexo plana ; umbonibus medianis ; latere postico convexiore, 

 angulo oblique subacuto ; margine anteriore et posteriore rectis, oblique declivibus ; basi 

 ellipticd curvatd. 



Shell ovately trigonal, elongated, rather depressed, pointed at the extremities ; umbones 

 mesial, depressed, small, and pointed ; the posterior side the more convex, with a subacute 

 oblique angle separating a space posterior to it, which is slightly concave ; anterior and 

 posterior margins straight, • and sloping obliquely downwards ; lower margin curved 

 elliptically. 



Specimens of this species present a considerable amount of variability in their 

 figure ; those from Lincolnshire are usually more convex posteriorly, and have the angle 

 more acute, the space posterior to it being somewhat concave ; the Minchinhampton 

 specimens are flatter, the umbones scarcely so much elevated, the posterior angle more 

 obtuse, the space adjoining it being flattened. These differences at first induced us to regard 

 the two as distinct species, and the first description of T. extensa, published in the 'Annals 

 of Nat. Hist/ for 1850, was deduced from Gloucestershire specimens, as compared with 

 the acute angle and otherwise distinct figure given in the ' Geology of Yorkshire ;' but an 

 examination of numerous specimens, and more especially of those from Lincolnshire, have 

 satisfied us that at the utmost, those of the North of England can only be considered as a 

 variety of the more common form seen in Gloucestershire. 



Tancredia angulata is a higher shell, with a shorter posterior, and more attenuated 

 anterior side. 



Length, 1 1 lines ; height, 6 lines. 



