BIVALVIA. 81 



OPIS SIMILIS, Sow., sp. Tab. VI, figs. 4, 4a. 



CAHDITA SIMILIS, Sow. Min. Con., p. 56, t. 232, f. 3, 1819. 

 OPIS SIMILIS, Morris. Catal., p. 96, 1843. 



Testa subrhomboided, fornicatd, concentrice lineatd, umbonibus terminalibus incurvis, 

 carind dorsali acutd, latere postico abrupto, lunuld pland. (Goldfuss.) 



Shell nearly rhomboidal or cordiform, elongated ; umbones terminal, rather augulated 

 and incurved ; dorsal surface with an elevated acute angle ; the lunule is very small and 

 cordiform, its borders rounded ; the anterior portion of the surface has concentric lines, 

 which pass over the carina, and are soon lost upon the flattened posterior surface. 



The height, measured along the dorsal carina, very much exceeds the lateral diameter, 

 the shell being much produced and pointed at the posterior and inferior extremity ; it is 

 associated with Opis lunulatus in the shelly beds of the Great Oolite, but is much less 

 common ; compared with that species it is much more lengthened and oblique, the lunule 

 minute, and the lines are much more delicate and closely arranged. 



Localities. Minchinhampton and Bisley Commons in the Great Oolite; Ancliff, 

 Wiltshire ; Ponton, Lincolnshire. Cloughton Wyke, Yorkshire. (Phillips.) 



OPIS DESHAYESII. Tab. VI, figs. 5, 5a. 



Testa dongatd, angustd, trapeziformi, concentrice costatd, antice depressd, postice acute- 

 carinatd, sublcevigatd, subsinuatd ; costis regularibus depressis ; lunuld magnd excavatd, 

 marginibus rotundis ; umbonibus elutis, angustis, incurvis. 



Shell elongate, narrow, trapeziform, the sides concentrically costated; anterior side 

 depressed, truncated ; posterior side acutely carinated, the carina separating a posterior 

 depressed and smooth area from the costated portion of the shell ; the posterior margin of 

 the shell forms an angle at its middle part ; lunule large and deep, its margins rounded ; 

 umbones elevated, angulated, and compressed at the sides. 



The general figure is compressed, elongated, and attenuated, irregularly pentagonal, the 

 anterior side being the most wide. The absence of an anterior angle is sufficient to 

 distinguish it from Opis 'cardissoides, Goldfuss ; but the two species which approach most 

 nearly to it are the Opis Archiaciana and 0. Michelinea, figured and described by 

 M. Buvignier in his work on the ' Geology and Palaeontology of the Department of the 

 Meuse -.' but in neither of the latter species does the convexity of the valves equal that of 

 our shell ; they are comparable to it in the elevation and attenuation of the umbones, but 

 are destitute of the regular concentric costae. 



Height, 5j lines ; opposite diameter, 3J lines ; diameter through both the valves, 4 lines. 

 Rare. 



Localities. Quarhouse, Bisley Common, and Minchinhampton Common ; Ancliff, 

 Wiltshire. 



11 



