318 Mr. W. Clark on the Pholadidse. 



under the umbones have long excited the attention of naturalists, 

 and the uses assigned to them as supports of the body, we be- 

 lieve to a certain extent to be correct ; but they have other im- 

 portant 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 be- 

 lieve 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 ascertained beyond doubt : 

 no muscle passes through the animal or embraces the mantle 

 anteally, the tough and thickened margins of which are sup- 

 ported 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 considered 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 position 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 claim 

 particular notice, as much error has existed with respect to them : 

 their utility originates from the two very strong muscular bands 

 given off from the foot, which are fixed to their spatulate rough- 

 ened terminations, and serve as points d'appui for its action in 

 the work of excavation, without endangering or distressing the 

 body. We have here a beautiful illustration of the resources of 

 nature to accomplish what is necessary for the well-being of her 

 creations ; for if the foot was attached to the body as in the ordi- 

 nary bivalves, without aid, the severe action of excavation would 

 probably paralyse the animal and tear it from its natural posi- 

 tion, which result is prevented by this admirable contrivance, 

 and the body lies securely supported in the curvature of these 

 appendages, whilst the movement of the foot is altogether main- 

 tained by working from the crotchets. 



Nature has not given this animal an anterior adductor, because 

 it is unnecessary, as it rarely expands the large gape by a sepa- 

 ration of the valves ; and when it does, the medial adductor and 

 the limited action of the ligament suffice : the permanent gape is 

 the equivalent for what in ordinary bivalves is effected by the 

 opening and closing of the shell by the adductor muscles. The 



