MOLLUSCA. 69 
forms these shells into two sections, the first containing 
those shells which have the muscular impressions separate 
and lateral, and the second such as have only one subcen- 
tral, simple, or compound impression. However highly we 
respect the conchological labours of this naturalist, we can- 
not join with him in the present instance, and elevate a sub- 
ordinate character to a primary rank. If, by muscular im- 
pressions, he means those marks impressed on the valves of 
the shells by the muscles which serve to close it, then his 
character is unconnected with any of the primary functions 
of the inhabitant. For is it of much consequence whether 
the valves be brought into contact by the action of one 
muscle or by the assistanceof two? In so far, the charac- 
ter is evidently artificial, when the impressions of the ab- 
ductor muscles only are employed. But he evidently uses 
the term in a more extensive sense, to refer to those im- 
prescions left on the shell by some of the other muscles by 
which the animal is attached to it. To the mere concho- 
logist, these marks are of a very uncertain import, and 
can never enable him to construct natural families, and the 
student of the mollusca will employ more important distinc- 
tions. If we are to take all the muscular impressions into 
account, the arrangement of Lamark must undergo great 
alterations. Let us take the common mussel as an exam- 
ple. It is placed by the French Conchologist in the second 
section, as having only one muscular impression, although 
no less than four muscles. adhere to each valve, destined for 
the performance of very different functions. The largest 
impression, which is situate near the obtuse end of the 
shell and towards the posterior margin, belongs to the ab- 
ductor muscle, employed in closing the valves. Connected 
