BIVALVIA. 7 



PLACUNOPSIS SOCIALIS. Tab. I, fig. 9, 9a. 



Testa parvd ovato-orbiculari, valvis valde incEqualibus et irregularibus, lineis radiantibus 

 subtillissimis confertis ; plicis concentricis paucis irreyularibus. 



Shell small, ovately orbicular, with the valves very unequal and irregular ; the 

 umbones are marginal but very depressed, and scarcely distinguishable ; the surface is 

 covered with extremely fine densely arranged radiating lines, which are commonly visible 

 under a magnifier upon the convex valve, and very rarely in the flat valve ; the concentric 

 plications are few, strongly marked and irregular. 



This little shell is usually coloured with tints varying from lake to indigo and brown ; 

 it occurs throughout all the shelly beds of the Great Oolite in the Minchinhampton 

 district ; and towards the middle of the series in the soft shelly Oolite or oven stone, it is 

 peculiarly abifndant and gregareous, the largest specimens having a diameter oi about 

 7 lines ; although it does not exhibit any marks of having been attached or compressed, 

 the figure of the valves is even more irregular than in P. Jurensis. 



Localities. Minchinhampton and Bisley Commons. 



PLACUNOPSIS ORNATUS. Tab. I, fig. 11,11. 



Testa parvd, ovato-orbiculari subpland, fragili, umbonibus sub-marginalibus depressis, 

 contulis radiantibus numerosis cqualibus et regularibus, aliisque interstitialibus tenuissimis, 

 costulis spinis fistulosis, numerosis, depressis ornatis. 



Shell small, ovately orbicular, transverse, compressed, irregular, very thin ; umbones 

 sub-marginal, depressed, radiating costse elevated, rounded, numerous, equal and regular, 

 with interstitial and very fine striae ; the costae are ornamented with numerous depressed 

 fistulous spines. 



The radiating costae are elevated and undulated, and the numerous depressed fistulous 

 spines which ornament them render it a pretty object under the magnifier ; the character 

 of the surface altogether is very similar to that of Ostrea spondyloides, (Schloth, Goldfuss, 

 t. 72, fig. 5,) but that species pertains to the Muschelkalk. The other valve has not been 

 recognised. 



Locality. Minchinhampton Common. 



PLACUNOPSIS RADIANS. Tab. I, fig. 10. 



Testa parvd sub-orbiculatd, umbone parvd, depresso, sub-marginali, lined cardinu 

 subrectd ; costulis radiantibus rotundis distantibus et fistulosis, interstilialibus l<evigatis ; 

 plicis concentricis distantibus. 



Shell small, sub-orbicular; umbo small, depressed, sub-marginal, hinge line nearly 

 straight ; radiating costa? rounded, elevated, rather irregular, distant, with fistulous 

 plications upon their surfaces ; the interstitial spaces are smooth ; the concentric plications 

 are few and distant ; the general convexity of the shell is moderate, but the convex valve 

 is unknown : not unfrequently near to the border a second series of costae commence, but 



