376. THE MEDALS OF CREATION. Cuap. XI. 
come within the scope of our inquiries: yet it is possible 
that the soft parts even of these perishable structures may 
have left some trace, or that markings of their integument 
on the silt or mud may be preserved ;* and I would recom- 
mend the student to search for such indications on the 
rippled surface of clays and sandstones. 
b. The Bracuiopopa (arm-feet) have two long spiral 
fleshy arms, or brachia, developed from the sides of the 
alimentary orifice, are enclosed in bivalve shells, and respire — 
by means of their vascular skin, or mantle. They have not 
the power of locomotion, but are fixed by a peduncle to 
other bodies. 
c. The LAMELLIBRANCHIA (plated gills) have also bivalve — 
shells, but their respiration is effected by gills composed of 
vascular membrane disposed in plates, and attached to the 
mantle ; the beard of the Oyster is the branchial or respi- 
ratory apparatus of that animal. These bivalve Mollusca 
are subdivided into those which close their shells by one 
adductor muscle, hence called monomyaria, as the Oyster ; 
and those which have two muscles, dimyaria, as the Cockle. 
As the impressions left on the shells, by the attachment of 
these adductor muscles, and by the margin of the mantle, 
are found as perfect in the fossil as in the recent, they con- 
stitute important distinctive characters. 
Dr. Gray’s definition of the respective parts of univalve 
yh 
and bivalve shells is at once clear, concise, and natural, — 
being conformable to the structure of the body of the © 
enclosed mollusk. 
The front of the shell is the part which covers the head of 
the animal ; the back of the shell is the part which covers 
the tail ; the left and right sides correspond with the same 
parts of the mollusk. . 
In univalves, the apex of the shelly cone whether it be 
* The Ischadites Kénigi of the Ludlow rock was es i to 
resemble Boltenia, a pedunculated Ascidian. 
