BIVALVIA. 69 



Shell ovately elongated; umbones placed anterior to the middle of the valves, rather 

 depressed and obtuse ; anterior side the shorter, its upper margin slightly excavated, its 

 lower extremity pointed ; posterior side larger, more convex, with an oblique obtuse angle ; 

 the hinge-border is moderately lengthened and horizontal ; the surface is smooth, the lower 

 border is elliptically curved. 



T. extensa, Lye., ' Gr. Ool. Mon.,' p. 93, approximates to the present form, but has a 

 much larger anterior side, with the umbones more elevated and mesial. 



The height slightly exceeds half the length. 



Geological Position and Locality. The Great Oolite of Kirklington, Oxon., collected 

 by Mr. Whiteaves. 



CORBICELLA SUB^QUILATERA, Lye. Tab. XXXV, fig. 12. 



COBBJCELLA suB^EQUiLATERA, Lycett. Cotteswold Hills Handbook, p. 126. 



Testa ovato-obliqua laviffata, umbonibus parvis, antero-medianis, lunula angusta, sulco 

 ligamenti angmto, margine superiore oblique curvato. 



Shell oblique, ovate, smooth ; umbones not prominent, placed a little anterior to the 

 middle of the valves ; anterior border slightly depressed, lunule narrow ; superior border 

 curved obliquely ; ligamental sulcus narrow and lengthened ; surface of the valves smooth, 

 the lines of growth being only faintly impressed. The height is equal to two thirds of 

 the length ; the diameter through both the valves is equal to about half the height. 



This shell presents an example of a remarkable series of Jurassic bivalves, whose cha- 

 racteristic features are intermediate between Corbis and Tancredia, and which may usually 

 be discriminated without reference to the hinge ; compared with Corbis, the more depressed 

 form, the smallness of the anterior side, and the surface destitute of ornament, will always 

 distinguish it ; from Tancredia by the more ovate form, and by the absence of the posterior 

 oblique angle. The hinge is figured upon PI. XII, fig. 15, of the ' Great Oolite Monograph ;' 

 but the artist has scarcely extended the hinge-lamina sufficiently to exhibit the depressed 

 posterior lateral lamellar process ; the absence of the anterior lateral tooth, and the figure 

 of the cardinal dentition, is also distinct from Corbis, and is more nearly allied to Tancredia, 

 from which it differs chiefly in possessing a lengthened hinge-lamina and depressed 

 remote posterior lateral tooth; these distinctive features are remarkably persistent in 

 every example of Corbicella, and tends greatly to strengthen its claims to a generic 

 distinction. 



Under the name of Corbis lucida our species was included in Mr. Bean's list of 

 Cornbrash Fossils, published in the 'Magazine of Nat. Hist.,' 1839, but was not accom- 

 panied by any figure or description. 



Geological Positions and Localities. The specimen figured is a fine example from the 



