466 MOLLUSCA. 
this point, towards the ventral margin, is a depressed, oval area, in 
which is a delicate fissure communicating with the interior; this area 
is usually covered by a membrane, so that the fissure is not seen till 
the membrane is removed; on the interior of this valve is a delicate 
median crest, rising into an apophysis, which is bifid at tip, at the 
base of which the fissure opens by a minute puncture. Upper valve 
having a more delicate median crest, on each side of which are the 
thickened scars for the attachment of the muscles; inner edge of the 
valves ciliated; at the hinge margin of the upper valve are two tuber- 
cles, forming a shelf, on which the edge of the lower valve rests. 
Diameter about one-third of an inch; width about one-eighth of an 
inch. 
A single specimen was found at Rio Janeiro, and partially examined, 
by Mr. Couthouy. It is apparently the same as the shells here 
described, which were collected by Dr. Geo. H. Perkins, at Cape 
Palmas, Liberia. ‘Their identity, however, is not certain, and the 
description must be considered as applying to the African shell. 
I have placed this shell in the genus Crania, in the first place, be- 
cause it seems to accord in most respects with the characters of that 
imperfectly understood genus; and secondly, because it is by no means 
clear that the genus Orbicula, the only other one to which it could be 
referred, is not essentially the same thing. ‘The only distinction 
which Deshayes makes between them is, that the latter has a pedicle 
of some length by which it is attached to foreign bodies, while the 
former adheres directly. This seems to be a frivolous distinction, 
and one impossible to be made out unless the animals can be carefully 
observed during life. Mr. Couthouy distinctly observed such a tendon. 
Lamarck gives the different texture of the shell as the distinctive cha- 
racter; but specimens of the above show various degrees of thickness, 
from thin, horny valves, almost destitute of muscular cicatrices, to 
those of massive thickness, with prominent cicatrices. ‘The little bifid 
apophysis is easily broken off, and therefore not always found; and 
the fissure in some specimens appears to be closed up, with only its 
vestiges remaining. 
Figures 580, 580 a, upper and under valves, enlarged ; 5808, edge 
view; 580c, interior; 580d, the tendon; 580e, natural size. 
