BIVALVIA. 19 
crassissimd, tatus irregulariter excavald ; extuslamellosd, valved supervore pland vel concard ; 
apice ad sinistram ? arcuato ; ared ligamenti latd. 
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 (Fordes). 
Belgium, Pietrebais, prés de Chapelle St. Laurent (JVys?). 
France, Roquencourt, le pare de Versailles (Desh.) 
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 does 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 (fig. 1, 4) curves 
towards the siphonilateral margin, while the specimen from Jermyn Strect (fig. 1, @) 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 coarscly laminated; the upper valve is much thinner 
than the lower, and flat, with the lamine finer and closer. 
An oyster from the Nummulitic Formation at Cutch is figured and described under the 
name O. callifera by Mr. Jas. Sowerby, ‘ Trans. Geol. Soe.,’ vol. v, pt. 2, second series, 
pl. 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 O. Sowerdyana. 
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’Archiae gives this species somewhat doubtingly from the Nummulitic Beds in 
the environs of Bayonne. 
5. Osrrea cyaTHuLa? Lawarck. ‘Tab. VII, fig. 7, and Tab. VIII, fig. 3. 
OstREA CYATHULA. Zam. Ann. du Mus,, t. viii, p. 163, No. 12, 1806. 
— —_ Desh. Coq. Foss. des Envy. de Par., t. 1, p. 369, pl. 54, figs. 1, 2; ana 
pl. 61, figs. 1—4. 
— — Id. An.s. Vert. du bassin de Par., t. u, p. 114, 1860. 
Spec. Char. O. “ testd ovato-rotundata, profundd, incrassatd, solida ; umbonibus magnis, 
posticé inflewis, aliquando contortis ; valvd majore subtis plicatd, plicis angustis, distantibus, 
radiantibus, lamellis transversis interruptis; valvd superiore pland, transversim striato-lamel- 
losd, superné crassa : ‘mpressione musculari seni-ovald, transversd ; fossula cardinali super- 
ficial, transversim striata.” 
