BIVALVIA. 21 



Gervillia subcylindrica. Tab. Ill, fig. 13, 13«, b. 



1 Var. of Gervillia acuta, Sow. 



Testa subcequivalvi, elongatd, subcylindrica, margine cardinali majis obliquo ; umbone 

 in valvd sinistra obliquo, prominulo, antico ; valves dextrce umbone depresso, acuto. Cardo 

 foveolis 3 aut 4, dentibus internis tribus obliquis, anticis. 



Shell subsequivalve, elongated, subcylindrical ; hinge line very oblique, lengthened, and 

 nearly straight ; umbo in the left valve oblique, prominent, but not terminal ; right valve 

 with the umbo depressed and acute ; hinge with three or four marginal pits, and three 

 oblique, anterior, and internal hinge teeth. 



Without care this shell may be confounded with G. aviculodes, Goldfuss, t. 115, f. 8, 

 but upon comparison the latter shell will be found wider, and the umbones straighter, and 

 more acute ; the hinge line also is inclined at a smaller angle, the general figure being 

 less elongated and cylindrical. From G. siliqua, Deslongchamps, the very different 

 inclination of the hinge line and straighter form will distinguish it. 



Locality. Minchinhampton Common, where it occurs somewhat rarely in the 

 planking. 



Gervillia Bathonica. Tab. II, fig. 15. 



Testa oblongd, planatd, subcequivalvi, umbonibus terminalibus acutis, margine cardinali 

 recto, obliquo, (plerumque ad unguium 45 gradum iuclinantibus,) latere antico recto interdum 

 subexcavato, basi curvatd ; foveolis (9) oblongis, cequalibus et regular ibus ; dentibus car dims 

 interne duobus, obliquis, anticis. 



Shell oblong, flattened, subasquivalve ; umbones terminal and acute; hinge line 

 straight, oblique, (for the most part inclined at an angle of 45 degrees,) anterior side 

 straight, sometimes rather excavated, base rounded ; hinge pits oblong (nine in number), 

 equal and regular ; internal hinge teeth two, anterior, and oblique. 



The figure varies so considerably, that but for the inspection of a considerable number 

 of examples of all stages of growth, they would probably be separated into at least two 

 species, hardly two specimens, indeed, can be found exactly alike in the figure of the 

 anterior margin, the angle at which the hinge is inclined, the degree of convexity in the 

 valves, or in the general length of the figure. 



The terminal position of the umbones together with the straight or even slightly con- 

 cave figure of the anterior margin gives to it very much the figure of Perna, but the large 

 oblique internal teeth in the hinge, and absence of the anterior hiatus or corrugation, 

 effectually separates it from that genus. 



Its habits were eminently gregareous, and in certain layers of the white stone at 

 Bussage and Eastcombs, it occurs in great numbers to the exclusion of nearly every other 

 species ; but even in these circumstances, the valves are very rarely found in apposition, 



