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 Gervillia acuta, Goldfuss; 

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

 distinction. 



Locality. Minchinhampton. 



Gervillia ovata. Sow., Sp. Tab. II, fig. 12, 12a. 



AvicuLA OVATA, Sow. Mill. Con., t. 512, f. 2, 182C. 



Testa ovato-obliqud, convexd et IcBvigatd, valvd sinistrd convewd, umbone ohliquo, 

 mediano, margins cardiwali brevi, subrecto, margine antico rotundo; auriculis submellis, 

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

 tantibiis et irregularibus ; dentibus cardinis interni subobsoletis. 



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

 hinge Une 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 difiierent, 

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



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, \^a, b. 



Gekvillia monotis, Deslongchamps. Mem. Soc. Linn. Calvados, torn, i, 1824, t. 5, f. 2. 



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

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

 depresso et acuta ; margine cardinali obliquo in auriculo 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 



