HIPPONYX. MONOMYARIA. 75 
Section 3. Adhering to marine bodies by their flattened valve. 
Genus XIV.— ORBICULA. — Cuvier. 
Generic Character. — Shell inequivalve, nearly orbicular, 
compressed, generally irregular in form, adherent, flat, and 
attached by means of a fibrous substance passing through 
an orifice near the centre of the lower valve; upper valve 
patelliform, its vertex posterior or nearly central ; each valve 
provided with four muscular impressions, two of which are 
large, approximate, and situate near the centre, and two 
smaller and more distant placed near the posterior margin ; 
those of the lower valve not so well defined as the others ; 
near the inner extremity of the orifice there is an obtuse 
testaceous process ; destitute of hinge teeth or a ligament. 
Orbicula granulata. Plate IX. fig. 10. Found in Argil- 
laceous Ironstone nodules in Alum Bay, Isle of Wight. 
The Orbicule are oceanic shells, consisting of only two or three species, in- 
habiting the European seas. 
Fossil species are limited, and met with in the Coral Oolite of Yorkshire, 
and the Great and Inferior Oolite of Wiltshire and Yorkshire. 
TRIBE I.— RUDISTA. 
Animal unknown, as are also the ligament and hinge ; shell 
with very unequal valves, aud destitute of distinct umbones. 
Genus XV.— HIPPONYX. — Defrance. 
Generic Character. — Shell bivalve, adherent, inequival ve, 
irregular; muscular impressions in both valves horse-shoe 
shaped; lower valve affixed to marine bodies, orbicular, 
much compressed, and considerably thickened in some in- 
stances, with its margins always elevated, particularly in 
front, its muscular impression consisting of two contiguous 
semilunar portions, which are distant, broad and rounded in 
front, nearly confluent and narrow behind; upper valve 
pateliiform, generally subconic, in some instances compres- 
sed, with a posteriorly submarginal umbo pointing back- 
wards ; muscular impression situate near the posterior 
margin, with its two lobes considerably more remote and 
obliquely truncated in front, but entirely confluent behind ; 
hinge destitute of a ligament or teeth, 
Hipponix cornucopia. Plate X. tig. 2, upper valve; and 
under valve, fig. 2.* 
The fibres in the muscular impressions are placed in a different direction 
from those of the other portions of the shell, and seem to be more liable to de- 
composition. In fossil species these present cavities, 
