BIVALVIA. 19 



young state, the shell is thin and delicate, more especially the right valve ; the latter 

 is consequently comparatively rare ; and M. Deslongchamps had not recognised it in 

 Normandy, when he published his description of Gervillia costatula in 1824 ; at that 

 period so few species of Gervillia were known, that the usual character of the surface could 

 not be considered as ascertained, nor likewise the limits to which any variation in the hinge 

 was restricted ; but now that a considerable number are recorded, it will be found that in 

 few instances where the hinge of Gervillia has been disclosed has a sculptured surface 

 been coincident with it : the hinge of Gervillia costatula given by M. Deslongchamps, 

 differs somewhat from our own, and from the figure of M. Buvignier ; but as it is still 

 more unlike the hinge of Gervillia, we might from the hinge alone conclude that it had 

 been incorrectly allocated. The shell figured by M. Buvignier, though very imperfect, is 

 readily recognised as our own species in the ultimate stage of growth, and having a degree 

 of obliquity greater than is usual. It occurs in all the shelly beds of the Minchinhampton 

 district, but specimens of the ultimate stage of growth have only been obtained in the 

 planking of Minchinhampton Common. The same species, or one nearly allied to it, has 

 also been procured, rarely, in the free-stone beds of the Inferior Oolite in the same 

 district. 



Localities. Minchinhampton ; Ranville near Caen, Normandy; St. Mihiel, France. 



Pteroperna pygmea, Bunker, Sp. Tab. II, fig. 11, 11 a. 



Avicula pygmea, Koch and Dunker. Norddeuts. Ool. Versteinerungen, t. 3, f. 6. 



Testa parvd, subaqiiivalvis, ovato-obliqud, concentrice idque obsolete striata ; aid 

 anticd rotunclatd, sinuatd, posticd excavatd ; umbonibus prominulis. 



Shell small, subaequivalve, ovately oblique, concentrically, but obsoletely striated ; 

 anterior wing rounded and produced, its lower border sinuated, posterior wing excavated 

 by a longitudinal furrow; umbones rather large and prominent, rising higher than the 

 hinge line. 



The right valve has a somewhat flatter surface than the other, and exhibits very 

 slight traces of one or two longitudinal costae ; the transverse lines are not usually pre- 

 served, and are visible only upon a portion of one of our specimens. 



Locality. It occurs somewhat rarely in the soft Oolite which overlies the Weatherstone 

 Beds at Minchinhampton Common. 



Pteroperna emarginata. Tab. II, fig. 10. 



Testa subpland, ovato-elongatd et obliqud, valvd sinistra laevigata, striis concentricis 

 subobsoletis ; aid anticd parvd, posticd emarginata. Valvd dextrd ignotd. 



Shell rather flattened, ovately elongated, and oblique; left valve smooth, with con- 

 centric and nearly obsolete striae; anterior wing small and pointed, posterior emarginated. 

 Right valve unknown. 



