70 MOLLUSC A. 



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 foot. 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. If we 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 



