ACEPHALA LAMELLIBRANCHIATA. 27 
extended into siphonal tubes; the external surface lines the 
shell, and is the constructor thereof, by the exudations of 
calcareous matter from its vessels; the inner surface embraces 
the body, and is hung—and this observation introduces those 
on the muscular system—by some inconsiderable muscles ; but 
the grand supports of its position, in all bivalves, are the ad- 
ductor muscles which pass through and suspend the body by 
bemg fixed to each valve, and when detached leave cicatrices, 
termed muscular impressions, which have been applied to assist 
generic and specific determinations. It is said that in the 
Anomie, Ostree, and Pectines, there is only one subcentral 
muscle: this is a mistake; as in those genera, as well as in 
the Myti and some others, I have found a second muscle, 
though often of very inconsiderable volume, under the beaks ; 
we may therefore consider the monomyal and dimyal charac- 
ters as untenable. The mantle in the Anomie, Ostree, and 
Pectines is entirely open from the beaks, and destitute of 
siphonal tubes; in others, as in Lucina, Mytilus, &c., it is 
more closed, and shows only two sessile orifices; in the fami- 
hes of the Veneride and Mactride a still further closure is 
apparent in that membrane, which in them is produced into 
two short tubes; in the Te/linide it forms rather long slender 
siphons; and finally, in the Pholades, Lutrarie, Mye, Gas- 
trochene, and the Sawxicave, it is entirely closed, except a 
pedal orifice, and is often produced into two long tubes, either 
soldered on each other or inclosed in a retractile sheath; the 
lower siphon is called the branchial, the upper the anal one; 
the former conveys water to the branchial cavity and expels 
it. We do not believe the doctrine that the water enters 
by one tube and is discharged from another; we think it 
is admitted both from the branchial siphon and pedal orifice, 
and expelled indiscriminately from those apertures. In many 
genera both, and in others only one of the siphons are pro- 
vided with valves at their orifices, to regulate the entry and 
expulsion of the water, and, with one or more circles of ten- 
tacular filaments, to capture and transfer the animalcule 
into the branchial cavity: on this point see our observations 
on the Pholades. 
