1 
tp-i 
hi '4 f 
t $(2 
£sr 
Crania, Retains. 
Gen. Char. An unequal valved bivalve, slightly 
irregular, suborbicular ; upper valve patel- 
liform ; lower valve attached, flattish ; mus- 
cular impressions four; no hinge; animal 
without a byssus. 
In this Genus the thickness of the lower or attached 
valve is very variable, even in the same species it is 
sometimes so thin as to have been overlooked ; never- 
theless it is that valve which possesses the characters of 
the Genus in the most eminent degree ; for, it is found 
to be attached, like the lower valve of Ostrea, by its own 
surface ; to be wholly destitute of hinge, by which the 
other valve is united to it, either in the form of teeth that 
lock into each other, or of ligament upon the edge ; to be 
marked with the impressions of four tendinous muscles, 
that unite it at once with the animal and the other valve ; 
and to have a granulated inner surface, especially about 
the margin, which arises from its cellular, and not foli- 
ated structure, a structure possessed also by Radiolites 
and Calceola, which Lamarck has arranged with it un- 
der his family “ Rudistes.” Of the four muscular im- 
pressions, two are placed at a distance from each other, 
near the margin, which is generally almost straight be- 
tween them ; the others are towards the centre of the 
disk, and only separated from each other by an eminence 
in the lower valve, while they are more distant in the 
upper ; the free or convex valve has a nearly central 
apex, is generally thin, and has the same granulated 
surface near the edge as the fixed valve has, but not in so 
remarkable a degree. 
Yo t, V, June 1823. 
