80 MOLLUSCA FROM THE GREAT OOLITE. 



and less homogeneous rock has enabled us to disclose the interior of the valves in many 

 instances. The valves occur of all sizes, both in conjunction and disunited ; the habits of 

 the species were gregareous to the almost entire exclusion of other Mollusks, a few casts of 

 Purpuroidea and Natica, however, accompany it. Pachyrisma occupies a vertical thickness 

 of only half a yard, and its horizontal extension would likewise appear to have been very 

 limited ; hitherto it has been found only at two localities of the same neighbourhood. 

 Locality. The vicinity of Minchinhampton and Chalford. 



OPIS. Defiance, 1825. 

 CARDITA, Sp., Sow., 1819. 



Gen. Char. Shell subtrigonal or cordate, thick, the valves convex, arched, the posterior 

 side being separated from the anterior by an angle or carina ; umbones prominent, large, 

 curved spirally outwards and forwards ; lunule large, cordiform, sometimes deeply exca- 

 vated. Hinge massive, the right valve with a large obliquely pyramidal tooth compressed 

 laterally, posterior to which is a narrow and deep cavity, with parallel sides ; the left valve 

 with a large subquadrate cavity to receive the tooth of the other valve, and a small 

 accessory tooth extending along the posterior margin. Ligament external. Muscular 

 impressions strongly marked and rounded ; palleal impression simple. 



OPIS LUNULATUS, Sow. var. Tab. VI, figs. 3, 3 a, b, c. 



CABDITA LUNULATA, Sow. Min. Con., p. 55, t. 232, f. 1, 2, 1819. 

 OPIS LUNULATUS, Morris. Catalogue, p. 96, 1843. 



Blainville. Malacol., t. 70 bis, f. 1, 



Testa triffond, ventricosd, concentrice lineatd ; umbonibus maynis involutis, carind dorsa/i 

 subacutd, elevatd, latere postico abrupte-plano ; lunuld cordatdprofundd, maryinibm actili*. 



Shell trigonal ; umbones large, angular, terminal, and curved outwards, the posterior 

 side bounded by a prominent and acute angled carina ; anterior side with closely arranged 

 regular concentric lines ; posterior side flattened or slightly excavated, smooth, or with 

 faintly-marked oblique lines ; lunule smooth, cordate, large, deep, its margins acute. 



This thick shell, with the valves disunited, is one of the most abundant bivalves of the 

 Great Oolite shelly beds ; the size of the lunule varies very much, as likewise does the 

 number and prominence of the concentric lines ; occasionally, indeed, the surface appears 

 to have become quite smooth in the more advanced stage of growth. 



The height and lateral diameter are of equal dimensions ; the diameter through both 

 the valves is one third less. 



Upon comparison with the typical form from Dundry, this variety is observed to be 

 smaller, less elongated, the lunule usually larger, and its margins more acute, but we do 

 not regard these differences of more importance than might be expected to occur in shells 

 procured from a different stratum and locality. 



Localities. The whole of the Minchinhampton district in the Great Oolite ; Ponton, 

 Lincolnshire. 



