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, 1(2. 



Osthea acuminata, Sow. Min. Conch., t. 135, f. 2 not 3, 1816. 

 Iioemer. Verst. Oolith., ii, p. 25, t. 18, f. 10. 



— — Bronn. Leth. Geog., p. 192. 



Testa ovato-elongatd, interdum subcrescenticd ; valvd inferior e convexa, umbone obliquo, 

 acuminate ; valvd superiore subpland, ovatd, tenui. 



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

 oblique, 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 Fuller'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, Sow. Tab. I, fig. 5, 5«. 



Ostbea costata, Soto. Min. Conch., t. 488, f. 3, 1825. 



— — Gohlf. Petref, t. 72, f. 8. 



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

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

 Knokrii, {Volt;.) Zieten Wurtt, lx, t. 45, f. 2. 



Testa parvuld, obliqud, ovali, valvd inferior e pro/ hindd costata; umbone qffixo, cost is 

 dichotomis radiatd ; superiore plana 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 little oyster." It is one of the miniature productions of the Ancliff 

 Limestone. 



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

 the Great Oolite of Ancliff, Wiltshire, and in Gloucestershire. 



