BIVALVIA. 3 



usually much larger, and the entire form is more irregular ; it may be regarded as forming 

 a passage between the crescentric figure of that shell and the less oblique species, with 

 radiately plicated surfaces and margins ; it was eminently gregareous, and most abundant 

 upon the flaggy beds or tile stones of the Forest Marble ; in the shelly beds of the Great 

 Oolite it is much less common, and the specimens are usually small. 



Localities. The vicinity of Tetbury and Cirencester in the Forest Marble ; Minchin- 

 hampton Common in the Great Oolite. 



OsTREA ACUMINATA, Sow. Tab. I, fig. 1, la. 



OsTKEA ACUMINATA, Sow. Mill. CoDch., t. 135, f. 2 not 3, 1816. 



— — Roemer. Verst. Oolith., ii, p. 25, t. 18, f. 16. 



— — Bronn. Leth. Geog., p. 192. 



Testa ovato-elongatd, inter dum suhcrescenticd ; valvd inferior e convexa, umhone ohliquo, 

 acuminato ; valvd superiore suhpland, ovatd, tenui. 



Shell ovately elongated, frequently subcrescentric, with concentric plications ; umbones 

 obhque, acuminated ; the smaller valve flattened, thin, ovate. 



This little species exhibits a full share of the varieties of form proper to the genus ; in 

 the marls and subordinate beds of hard rag in the FuUer's-earth, the valves constitute a 

 considerable portion of the entire deposit, and in the shelly beds of the Great Oolite it 

 occurs in great profusion ; in France and the Jura it occupies a similar position, and in 

 equal prominence. 



Localities. Bath, Minchinhampton, and numerous other localities throughout the 

 course of the Fuller's-earth and Great Oolite. 



OsTREA cosTATA, 8ow. Tab. I, fig. 5, 5a. 



OsTBEA COSTATA, Sow. Min. CoDch., t. 488, f. 3, 1825. 



— — Goldf. Petref, t. 72, f. 8. 



— — Bronn. Leth, Geog., p. 190, t. 18, f. 18, 1851. 



— — Beshayes. Traite Element., t. 53, f. 10—12, 1850. 

 Knoebii, (Volts.) Zieten Wurtt., Ix, t. 45, f. 2. 



Testd parvuld, obliqud, ovali, valvd inferiore profundd costatd ; umhone qffixo, costis 

 dichotomis radiatd ; superiore pland subradiatd. 



Shell small orbicular, or obliquely oval, the attached valve deep with numerous 

 branched and somewhat rounded ribs, upper valve flat, margin undulated. 



Mr. Sowerby remarks that the "branching, rounded ribs upon the under surface 

 define this neat httle oyster." It is one of the miniature productions of the Anclifi" 

 Limestone. 



Localities. In the Cornbrash and Forest Marble of Wiltshire and Somerset ; and in 

 the Great Oohte of Anchff", Wiltshire, and in Gloucestershire. 



