20 MOLLUSCA FROM THE GREAT OOLITE. 



The degree of obliquity and flatness is much greater than in the associated species, 

 and the anterior wing is smaller ; some traces of an elongated groove are visible upon the 

 posterior wing of the left valve. 



Locality. Minchinhampton Common, where it is very rare, and occurs in the bed of 

 soft shelly Oolite which overlies the Weatherstones. 



Gervillta, Defrance, 1820. 



Gen. Char. Shell subaequivalve or inequi valve, inequilateral, elongated, transverse ; 

 hinge margin usually straight, lengthened, and oblique ; umbones small, oblique, anterior, 

 contiguous, rarely terminal ; hinge linear, marginal, with many oblong parallel, but rather 

 distant and irregular pits in each valve, placed transversely to the hinge line ; internal 

 hinge teeth parallel, oblique, placed anteriorly, or beneath the transverse sulci. 



The increased number of species exhibit characters which render it necessary to 

 arrange them under two sections. 



a Shell subaequivalve, margins of the valves regular. 



b' Shell very inequivalve and irregular, more or less contorted ; margins of the valves 

 close fitting, but sinuated. 

 Section a' comprises the usual well-known subaequivalve species. 

 Section b' has for examples G. monotis, Deslong., G. Hartmanni, Goldf., and 

 G. tortuosa the Gastrochcena tortuosa of Phillips. 



Section a'. 



m 



Gervillia acuta. Sow. Tab. Ill, fig. 12, 12a. 



Gervillia acuta, Sowerby. Min. Conch., t. 510, fig. 5, 1826. 



— LANCEOLATA, Gold/uss. Pctref., t. 115, fig. 9. 

 ? — ACUTA, Phillips. Geol. York., tab. 9, fig. 36, 1835. 

 ? — siLiQUA, Deslongchamps. Mem. Soc. Linn, Calvados., t. 4, 1824. 



Testa lanceolatd in vertice convexd, marpine cardinali perobliquo depresso, umbonibus 

 angustis acutis foveolis (3) quadratis. (Goldfuss.) 



Shell lanceolate, slightly curved, subaequivalve, anterior side moderately convex, 

 posterior side compressed and attenuated; hinge margin very oblique, with three 

 quadrate hinge pits ; umbones attenuated, anterior auricle acute and pointed, posterior 

 auricle forming an obtuse angle with the border beneath it ; folds of growth irregular, 

 strongly marked. Compared with Gervillia tnonotis, the valves will be found less 

 contorted or more nearly equivalve, the hinge border shorter, and the posterior auricle is 

 not produced as in that species ; in common with other species, the right valve is thinner, 

 smoother, and more flattened than the other. 



Localities. It occurs in the Stonesfield slate of the Cotteswolds (Buckman). In the 

 slaty Oolite of Collyweston, Northamptonshire. 



