BIVALVIA. 5 



This species is considered to be distinct from the 0. acuminata, which is more regular, 

 symmetrical, and incurved. It is very abundant in certain marly deposits belonging to 

 the upper portion of the Great Oolite in Northamptonshire. 



Localities. Sharnbrook and near Bedford ; Blisworth, Kingsthorpe, Yardley and 

 Aynhoe, Northamptonshire; Sapperton, Gloucestershire. 



Sub- Genifs EXOGYRA. 

 Shell with the umbones involute. 



EXOGTRA AURIFORMIS, Goldfuss. Tab. I, fig. 7. 



EXOGYKA ACKIFORMIS, Goldfuss, Petref., t. 86, f. 5 



Buckman. In Geol. of Cheltenham, p. 69. 

 ? OSTREA OBSCUKA. Sow. Min. Con., t. 488, f. 2? 



Testa ovato-suborbiculari depressd, valvd minore pland, inferiore subconica ; umboni- 

 bus minutis involutis ; laminis concentricis tenuissimis irregularibus. 



Shell ovately-orbicular, depressed, the smaller valve flattened, the larger subconical 

 with a large adhering surface ; umbones very small and involute ; laminae of growth con- 

 centrical, very delicate, and irregular. 



This pretty species is sufficiently distinct from the contemporaneous species ; it was 

 collected by Professor J. Buckman, in a bed of yellow clay at Sevenhampton Common, 

 and he has kindly placed it at our disposal. 



Locality. Sevenhampton Common near Cheltenham. 



PLACUNOPSIS. Nov. Gen. 



Testa suborbiculari, inesquivalvis, irregulari, tenui, non-auriculatd ; valvd major e con- 

 vexd, subobliqud, umbone depresso, submarginali ; lineis radiantibus undulatis ornatd ; 

 margine cardinali brevi subrecto. Valvd minore pland Integra, interdum affixd. Cardo 

 dentibus nullis, foved parvd transversd intend. Impressio musculari magno (biloba) ? 

 elliptica, subcentrali. 



Shell suborbicular, inequivalve, irregular, very thin, without ears; the larger valve 

 convex, rather oblique, its umbo depressed and subrnarginal, the surface ornamented with 

 undulated radiating lines ; hinge-margin short, nearly straight. The smaller valve is flat > 

 destitute of any foramen, and not unfrequently is affixed by its surface to other bodies. 

 Hinge without teeth, with a small mesial transverse internal groove to contain the 

 ligament. Muscular impression large (bilobed?), elliptical, subcentral. 



This genus in its figure and character of the surface presents a considerable resem- 

 blance, both to Anomia and Placuna, but although possessing certain features of affinity to 

 each of these forms, it is not the less separated from them by other characters of some 

 importance. It is so irregular, that scarcely two specimens have exactly the same figure, 

 so that the longer diameter may be either lateral or otherwise ; notwithstanding this 

 irregularity, however, it will be observed that the posterior or left side of the convex valve 



