ORBICULA Lamarck. 
(Discina, Lamarck.) 
Gen. Cuar*. “ Bivalve, inequivalve, nearly orbi- 
cular, compressed, fixed ; upper valve patelli- 
form, with 4 internal muscular impressions, two 
rather large and approximating near the cen- 
tre, and two smaller and more distant, placed 
near the posterior margin. Lower valve flat, 
with corresponding muscular impressions and 
a rather obtuse process placed at the inner end 
of a fissure near the centre. Hinge none.” 
ee 
“Te animal has two ciliated arms or tentacula, and 
adheres by a muscle or ligament which passes through 
the fissure.” 
Little can be added to the above detailed Gen. Cha- 
racter. The shells that compose the genus are thin and 
rather coriaceous. The animals contained in them are 
analogous to those of Terebratule and other Branchio- 
poda; but the foot or byssus, instead of passing through 
the beak of one valve as in Terebratula, and being in- 
closed in a tube, passes through the disk and spreads im- 
mediately upon the stone it is attached to. 
Two recent species are known ; they are described, in 
the thirteenth volume of the Linnean Transactions, by 
my brother : and we have now the pleasure of adding 
three fossil ones, two of them from the collection pur- 
chased by him from the late Mr. G. Humphries, whose 
merits and penetration have unfortunately been rendered 
of little service to the scientific world. 
* Trans. Linn. Soc. vol. 13. p. 466. 
