22 MOLLUSCA FROM THE GREAT OOLITE. 



and the usual length is about an inch ; it occurs, however, in all the shelly beds of the 

 Minchinhampton district. 



In examining approximate species it will be found that the figure is less quadrate than 

 Perna quadrata, Phillips ; the terminal umbones separate it from Gervittia acuta, Goldfuss; 

 and from Perna mytiloides, Goldfuss, the straightness gf the hinge line is a point of 

 distinction. 



Locality. Minchinhampton. 



GBRVILLIA OVATA, Sow., Sp. Tab. II, fig. 12, 120. 



AVICULA OVATA, Sow. Min. Con., t. 512, f. 2, 1826. 



Testa ovato-obliqud, conveccd et lavigatd, valvd sinistrd convexd, umbone obliquo, 

 mediano, margine cardinali brevi, subrecto, margine antico rotunda ; auriculis submellis, 

 valvd dextrd plano-convexd, umbone parvo. Sulcis cardinis eccterne tribus, magnis, dis- 

 tantibus et irregularibus ; dentibus cardinis interni subobsoletis. 



Shell ovate, oblique, convex, smooth ; left valve with the umbo oblique, nearly mesial ; 

 hinge line nearly straight, short, the anterior extremity rounded, auricles small ; right 

 valve more flattened, the umbo small ; external sulci of the hinge three, large, wide, distant 

 and irregular ; internal hinge teeth scarcely distinguishable. 



The valves are always separated and delicate, the test being usually but imperfectly 

 preserved; the younger specimens are shorter in proportion, and the hinge line exceeds 

 half the length of the valves ; but in others of large dimensions it is less than half the 

 length. Owing to its delicacy it is seldom that the hinge can be exposed, but inde- 

 pendently of this, it entirely wants the anterior hiatus and corrugation which exists in 

 Avicula and Perna, from the latter genus, indeed, the character of the surface is different, 

 and it does not possess the squamous structure of the Perna. 



Localities. The whole of the Minchinhampton district, in the shelly beds of which 

 it is moderately common, more especially about the middle of the shelly series. Also at 

 Stonesfield, Oxfordshire. 



Section V. 



GERVILLIA MONOTIS, Deslongchamps. Tab. II, fig. 14, 14, b. 



GERVILLIA MONOTIS, Deslongchamps. Mem. Soc. Linn. Calvados, torn, i, 1824, t. 5, f. 2. 



Testa elongatd, subarcuatd, valvd sinistrd convexd ; umbone antico, subterminali ; 

 laminis incrementi impresses ; valvd dextrd pland, interdum concavd, umbone terminali, 

 depresso et acuto ; margine cardinali obliquo in auricula postico acuto,producto. Cardine 

 foveis (5) subremotis, dentibus interni duobus, magnis, anticis. 



Shell elongated, somewhat bow shaped, left valve convex; umbo anterior, oblique, 

 almost or completely terminal, prominent, and impressed with laminae of growth ; right 



