BIVALVIA. 89 



In none of the Oolitic forms do we find a greater variety of figure than in this species, 

 and without ample materials for comparison, its examples would probably be regarded as 

 pertaining to more than one species ; these variations, which are irrespective of growth, refer 

 to the degree of convexity, the extent to which the valves are produced posteriorly, and the 

 more or less compressed and angulated, or, on the other hand, rounded and convex figure 

 of the posterior side of the shell. The valves occur in such considerable numbers, and so 

 fully illustrate all these minor variations of figure, as to remove all doubt that they belong to 

 the same species, even though we place together two examples of very dissimilar aspect. 

 The shell is rather thin, always very fragile, except at the umbones, which are not un- 

 frequently the only portions preserved when the shelly beds are more than usually detrital 

 in their character. The valves rarely occur in contact ; but when this happens the ligament 

 is preserved. 



The subjoined proportions must be regarded as representing the median figure of the 

 species. Height, 13 lines; lateral diameter, 15 lines; diameter through both the valves, 

 10 lines. It ranks as one of the most abundant of the bivalves in the Minchinhampton 

 district, and ranges throughout the shelly beds. Named after J. G. Lowe, Esq., who has 

 assiduously collected an interesting series of fossils from the middle Oolite. 



Localities. Minchinhampton Common ; Bisley Common. 



Ctprina trapezifooiis, et var. stjbrottjnda. Tab. XIII, fig. 5, 5a, c. 

 Venus tkapeziformis, Roemer. Verst. Nordd. Oolith., t. 7, fig. 14. 



Testa orb imlato-subtr ap ezif ormi > convexo-pland ; antice rotundatd ; postice subproductd, 

 angulo acuto carinato-depresso ; umbonibus anticis incurvis. 



Shell orbicular or subtrapeziform, moderately convex ; anterior side rounded ; posterior 

 side somewhat produced, forming a depressed angle ; umbones anterior, incurved. 



This small species occurs abundantly throughout the shelly beds of the formation at 

 Minchinhampton, with the valves disunited. When well preserved, its surface exhibits 

 concentric, irregular, and very fine striations ; it is shorter and more convex than C. Loweana. 

 The form which we have designated as a variety has greater convexity, and the posterior 

 side has not the .angulated outline of the typical form. 



Dimensions of this variety: height, 8 lines; lateral diameter, 9 lines; diameter through both 

 the valves, 7 lines. Another line added to the lateral diameter will represent the typical form. 



Localities. Minchinhampton Common ; Bisley Common. 



Ctprina jurensis, Gold/., sp. Tab. XIII, fig. 3. 



Venus jurensis, Goldfuss. Petref., p. 245, t. 150, fig. 17. 

 Testa parvd suborbiculari ; umbonibus mediants minutis ; lunula ovatd ; area 



lanceolatd. 



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