Some new Upper Greensand Bivalves. 117 



tents of his drawers at my disposal ; Mr. Liveing (M.A., St. Jolin's 

 Coll.) favoured me with all his rarer bivalves; and l)r. Cookson, 

 whose cabinet has always been accessible to me, procured from 

 Dr. Guest the loan of some beautiful specimens. To all these 

 gentlemen I oiFer my sincere thanks. 



Ostrea cunabula. PL V. fig. 1. 



Shell oblong, slightly ovate. The right valve nearly flat ; the 

 left deeply hollowed like a cradle, the excavation extending 

 under part of the hinge area ; in general the interior line 

 which supported the upper valve is remarkably rectangular. 

 Hinge area triangular, bordered by a continuation of the 

 collar, which projects above and surrounds the upper valve. 

 Under side rugosely laminated and plicated in the minor 

 degree. Upper valve excentrically and closely laminated ; 

 sometimes rather convex, and always more or less hollow in- 

 ternally between the hinge and muscular impression. Mus- 

 cular impression on the left side ovate, below the middle of 

 the shell, and touching the marginal thickenings. It is 

 shallow and nearly smooth. 



In the young state the shell appears, as seen behind the umbo 

 of one specimen, to have been a little flat circle indistinguish- 

 able from the countless small things which occur on every fossil 

 and on almost every nodule the bed contains. It is generally 

 partly attached, and sometimes becomes deformed from want of 

 space. One specimen commenced growth in the interior of a 

 broken Radiolite, and impressed its shell into the prismatic 

 cavities. 



I know no Cretaceous Oyster with which this may be com- 

 pared. It has relatives both in the Purbeck and Eocene, but 

 too distant to require distinguishing. 



Rare; gregarious. Coll. Dr. Guest, Dr. Cookson, IMr.W. Farren, 

 and University. 



Loc. Fen-Ditton and Cambridge*. 



Ostrea lagena. PI. V. fig. 2. 



Ovate, narrow across the hinge area. The right valve flat, 

 marked with very fine lines of growth ; the left attached 

 (throughout its length) and hollow, the hollow extending 

 slightly under the hinge area. The right side is developed 

 upwards, while the left is but a little thickened, thus makhig 

 the (adult) shell wedge-shaped ; the edge of the lower valve 

 projects above the upper. Hinge area subquadrate, and bor- 

 dered by a continuation of the projecting fringe. When the 



* Where no locality is menfioned, the specimens are obtained from the 

 immediate neighhouihood of Cambridge. 



