180 PHOLADHLE. 



this posterior ligament is to assist in maintaining 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, .1 think I have ascertained 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 ligament, 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 



