180 PHOLADID Ai. 
this posterior ligament is to assist in maintaiming the valves 
in their natural position. It appears then that Pholas is iron- 
bound as to ligament, which, in it, is far more powerful in 
securing the valves than in the shells of any other group of 
the Acephala of similar fragility and tenuity. 
The Muscular System. 
It will now be convenient to notice the muscular system ; 
and in the first place, that part of it connected with the shell. 
In this group of bivalves, the curved spatulate apophyses 
springing under the umbones have long excited the attention 
of naturalists, and the uses assigned to them as supports of 
the body, we believe to a certain extent to be correct; but 
they have other important functions that have not attracted 
sufficient notice. Before they are mentioned, it will facilitate 
their illustration if it is now stated, that though an anterior 
adductor muscle is spoken of by authors, there is not a trace 
of one in Pholas dactylus, and I believe all its congeners are 
also deprived of this organ. 
There is only one adductor muscle in Pholas, not posteriorly 
situated, but very slightly post-medial: the fact of the absence 
of this organ anteriorly, I think I have ascertamed beyond 
doubt: no muscle passes through the animal or embraces the 
mantle anteally, the tough and thickened margins of which 
are supported on both sides the shell and around the gape by 
long thin strap-shaped fillets thrown off from the medial 
adductor muscle, which, with that of the foot, may be con- 
sidered as the great points of departure of all the principal 
muscles of the body. 
Some authors contend that what I call the hgament, under 
the dorsally reflected mantle, is the anterior adductor: this 
idea cannot be supported, as independent of this strange posi- 
tion for an adductor muscle, the two layers of filaments are 
fixed, the one external to the other, with a space between 
them, to the shell, and not to the animal; therefore they are 
ligamental, and their action and reaction have the same effect 
as in the ordinary bivalve ligament. 
The important functions of the crotchets under the beaks 
