ON MOLLUSCA. 173 
than one. They are deeper, and result as well from the attach- 
ment of the ligamentous fibres as of the muscles. 
We shall see by and by that the first, which have so much 
analogy with the epidermis, have not less with the dried mus- 
cular fibres of the byssus. Accordingly, the impressions which 
they leave upon the shell are absolutely of the same aspect ; 
but two sorts have been observed, one external, or extero- 
internal, more or less elongated, occupying the dorsal part of 
the valves behind, and very rarely in front of the summits; the 
other, entirely or almost altogether internal, usually rounded 
under the summits, as in mactra, crassatella, &c. 
The impressions produced by the muscular fibres are much 
more numerous. They may be divided into those of the ad- 
ductor muscles, of the retractor muscles of the foot, of the 
attachment of the edges of the mantle, and finally of the 
attachment of the tubes. 
The impression of the adductor muscles is sometimes sim- 
ple, or unique, central or not, as may be seen in the ostracea 
or sub-ostracea, and in the pholades. It is sometimes sub- 
divided, as in anomia. 
It appears again to be unique in the mytilacea, but on con- 
sidering it attentively, we see, altogether in front, a very small 
impression, which is the commencement of the double muscu- 
lar impression, that is found in almost all the lamellibranch 
acephala, one part of which is buccal and the other anal. 
The form, the proportion, and even the position of these two 
impressions, vary very much, and furnish good characters for 
conchology. 
The impressions of the retractor muscles of the foot are 
always much smaller, and are frequently confounded, espe- 
cially the posterior ones, with those of the adductor muscles, 
where they form a sinuosity. They are numerous in the 
mytilacea ; in the conchacea, the anterior one, alone distinct, 
ascends over the hinge. 
The impression of the edges of the mantle, and that of the 
