LAMELLIBRANCHIATA. 103 



Jurassic rocks, but which are very characteristic of the Cretaceous 

 series. A number of names have been given to different varieties 

 such as 0. diluviana, macroptera, carinata, larva and rectangularis, 

 all of which names distinguish well-marked types more or less local- 

 ized in their geological and geographical distribution, but which are 

 found to pass into one another through intermediate forms when a 

 large series is examined. The relation of the types to one another 

 is indeed well illustrated by the arrangement under Prof. Geinitz in 

 the Dresden Museum where the well-marked types are arranged 

 on tablets and a large tray of intermediate forms is also exhibited. 

 The Upware forms belong decidedly to the variety macroptera 

 (Sby.), which is distinguished by its umbonal region being expanded 

 to form a wing, especially on the posterior side. 



Localities. Upware, Brickhill, Potton, Farringdon. 



N. Europe. Hils formation, Schb'ppenstedt. 



S. Europe. France and Switzerland. 



Ostrea frons, Parkinson, var. carinata, Sowerby, Min. Con., 

 pi. 365. 



Occurs at Brickhill, Atherfield, etc. 



OSTREA WALKERI, n. sp. 

 (Plate IV., fig. 4, a, 6.) 



A variable oyster shell one of the Rudes, plano-convex or con- 

 cavo-convex, covered with rude, imbricating growth-lamellae : the 

 shells are moderately thick. It is attached sometimes by the 

 whole surface of the lower valve but in other specimens by only a 

 very small area ; the lower valve in the former case has a thickened 

 margin. Contour ovate more or less irregular, beak pointed. 



Lower valve moderately deep, chamoid, or shallow. Upper 

 valve flat or concave, and irregular with concentric growth striae or 

 imbrications, which are usually less prominent than on the lower 

 valve. 



Ligament area large and triangular (about th length of the 

 whole shell), its base usually broadest. A median gutter serves to 

 divide it into three sub-equal parts; in the upper valve the liga- 

 ment area is much smaller. 



