BIVALVIA. 19 



crassissimd, intiis irregulariter excavatd ; extus lamellosd, valvd wipe, -lore plana rrl ooncavd ; 

 apice ad sinistram ? arcuaio ; area ligamenti lata. 



Shell ovate, with a projecting callosity on the opposite side to the curved umbo ; shell 

 thick, irregularly excavated ; externally lamellated ; upper valve flat. 



Dimensions, 4 inches by 3. 



Localities. Hempstead, Isle of Wight {Forbes). 



Belgium, Pictrebais, pres dc Chapelle St. Laurent {Ngst). 

 France, Roqucncourt, le pare de Versailles {Desk.) 



This species, so far as I know, is in England confined to the upper beds of the older Ter- 

 tiaries called the Hempstead series, and there it docs not appear to be abundant. Its peculiar 

 character is the callosity on the left or lower valve, caused by the broad adherence of the 

 animal inclining to one side ; this habit is retained until it is considerably advanced in age. 

 The shell is thick and heavy; the umbo of Mr. Edwards's specimen (tig. 1, b) curves 

 towards the siphonilateral margin, while the specimen from Jcrmyn Street (fig. I, a) has 

 the umbo in the opposite direction. This depends upon the mode of attachment in the 

 young state, the umbo having been deflected by an impediment. The outer surface of the 

 lower valve is irregularly rugose and coarsely laminated ; the upper valve is much thinner 

 than the lower, and flat, with the laminae finer and closer. 



An oyster from the Nummulitic Formation at Cutch is figured and described under the 

 name 0. callifera by Mr. Jas. Sowerby, ' Trans. Geol. Soc.,' vol. v, pt. :2, second series, 

 pi. xxv, fig. 16. The Cutch specimen, which I have examined in the Museum of the 

 Geological Society, does not, I think, belong to this species, and M. D'Orbigny, in his 

 ' Prod.,' has named the shell 0. Sowerbgana. 



The upper valve of an oyster from Uffhofen in the Museum of the Geological Society 

 is marked with this name, but it would be difficult to determine a species from that valve 

 alone ; the specimen is peculiar in being excessively thick, and it is perforated in the centre 

 by the entire abstraction of the shell where the adductor muscle was attached. 



M. D'Archiac gives this species somewhat doubtingly from the Nummulitic Beds in 

 the environs of Bayonne. 



5. Ostrea cyatiiula? Lamarck. Tab. VII, fig. 7, and Tab. VIII, fig. 3. 



Ostrea cyathula. Lam. Ann. du Mus., t. viii, p. 103, No. 12, 180G. 



— — Desk. Coq. Foss. des Env. de Par., t. 1, p. 369, pi. .34, figs. 1, 2 ; and 



pi. 61, figs. 1 — 4. 



— — Id. An. s. Vert, du bassiu de Par., t. ii, p. 114, 1860. 



Spec. Char. 0. " testa ovato-rotundatd, profunda, incrassatd, solidd ; unihonibus magnis, 

 postice inflexis, aliquando contortis ; valvd major e subttis plicatd, plicis angustis, distantibus, 

 radiantibus, lamellis fransversis interruptis ; valvd superiors plana, transversim striato-lamel- 

 losd, super ne crassd : impressione musculari semi-ovatd, transversa ; fossula cardinali super- 

 ficiali, transversim striata.'' 



