70 MOLLUSCA. 
with this impression there is a tongue-shaped mark, reach- 
ing nearly to the ligament. This mark is occasioned by 
one of the lateral muscles for supporting the byssus, and by 
one of the lateral muscles of the feot. The other muscle 
for supporting the byssus, is inserted under the teeth which 
occur at the beak. There is even another mark of adhe- 
sion on the margin of the shell, irregularly denticulated, 
occasioned by the fringed margin of the cloak, which is 
there united with the shell. This mark may be termed the 
marginal impression. ‘To which of these marks then are 
we to attach the greatest importance? To the impression 
of the one abductor muscle, which is common to all shells, 
—to the marks of the muscle of the byssus, or to the in- 
dented mark of the fringed margin of the cloak. Ifwe at- 
tend farther to Lamark’s arrangement, we shall find the 
Camacea separated from the Ostreacea, although the two 
families possess numerous external and internal points of 
resemblance. We regard the muscular impressions as fur- 
nishing a convenient character for the construction of genera, 
and the discrimination of species, but it is not worthy to 
occupy so high a rank as Lamark has assigned to it. 
As intimately connected with the muscular impressions, 
we may here notice the ligament. It is a horny elastic 
membrane, which serves to open the valves, when the ad- 
ductor muscle relaxes. It is placed on the exterior margin 
in some shells, and is concealed in others. When external, 
it is stretched when the shell is closed, and when it is inter- 
nal, it is compressed in similar circumstances. This cha- 
racter is very useful in the construction of genera, but it’ 
ought never to be employed in any of the higher divisions. 
It is not the index of any peculiar organization, neither does 
