186 SUPPLEMENT 
which is attached to one of the valves in such a manner as by 
their contraction to approximate one to the other. These 
are the adductor muscles; sometimes they form but a single 
mass drawn together in the middle of the valves, at other 
times the mass tends to subdivide into two or three; finally, 
in a great number of cases there are two very distinct, one 
anterior and the other posterior, the form and proportion of 
which, moreover, are variable. It is the insertions of these 
muscles in the valves of the shell which form what are named 
the impressions or the muscular attachments, in the same 
manner as it is that of the edges of the mantle which forms at 
the inferior and posterior edges of the shell a line more or 
less broad, more or less sinuous, or retracted backwards, 
which we have already noticed in treating of the shell, and 
the consideration of which is not without importance in con- 
chology. 
In the locomotory apparatus of the bivalve mollusca we 
ought also to consider the mode in which the pieces of the 
shell unite, or their system of articulation, and especially 
the ligaments, which serve not only to retain them in a 
determinate relation, but also to act as antagonists of the 
adductor muscles. On the first we shall make a few remarks 
when we come to speak a little more on conchology. As to 
the ligaments, we may add here, that, formed of a corneous 
substance, evidently epidermic, they are composed of trans- 
verse fibres which pass from one valve to the other, just as do 
the contractile fibres of the adductor muscles, and which have 
much resemblance to those which constitute the dried part of 
the true byssus, and still more of the tendinous foot of the 
tridacne. 'The ligaments observed in the shell of the ace- 
phalous mollusca may be distinguished into epidermic, ex- 
ternal and internal. ‘The epidermic ligament is that which 
is formed by the epidermis itself of the valves, which is con- 
tinued in passing from one to the other, as in solens, &c. 
