496 TRANSACTIONS OF THE WAGNER FREE 



largely due to the protection afforded by the burrowing habit universal in the 

 group. The small teeth, which hardly add to the efficiency of the hinge, may 

 be supposed to be associated with the submersion of the resilium and the de- 

 velopment of a chondrophore, as supposed by Neumayr, but in some cases 

 they may be the remnants of hinge-teeth acquired in the ordinary way in the 

 early geological history of the group. 



The Pholad Myophore. — In the most specialized group of all Pelecy- 

 pods, the Pholades, a remarkable development of the subumbonal attachment 

 of the mantle has produced a myophore which is sometimes wrongly re- 

 ferred to as a tooth and which in Bar?iea costata and Zirplicea crispata sup- 

 ports the visceral mass and the enormous fleshy palpi. The exceptional 

 development of this feature is explained by the dynamics of Pholad existence. 

 When we remember that the enormous foot is chiefly employed, during growth, 

 in grinding away the sides of its burrow, and that its action for a long period 

 is ince.ssant and vigorous, it will be realized how important it is for the welfare 

 of the animal that the vLsceral mass should be well supported and protected 

 against the effect of sudden motions such as, in other moUusks, might en- 

 danger its attachments to the valves. It is not surprising that natural selection 

 should promote the development of any shelly process which is of use in 

 affording this needed support. The initiatory stages of the myophore, as well 

 as of the reflection of the anterior dorsal border of the valves, can be seen in 

 Gastrochcena, especially if a thickened senile specimen can be examined. It 

 may not be out of place here, while speaking of the Pholads, to point out the 

 dynamic influences which have promoted the reflection of the dorsal anterior 

 margin. Observation as well as theory shows, that in order that the rasping 

 action of the foot shall expend itself on the walls of the burrow, it is necessary 

 that the shell as fulcrum should be fixed ; otherwise the animal, instead of 

 being able to rub the foot over the wall, would rub its valves about, while fric- 

 tion would hold the foot relatively still. In ordinary cases the mere elastic 

 tension of the ligament and the relaxation of the adductors would be all that 

 could be reckoned upon to hold the valves against the walls of the burrow. 

 Now if we should have an animal with the anterior adductor so situated on 

 the dorsal margin as to be able to give a positive pull on the valves, so as to 

 hold them open against the walls, instead of a merely negative relaxation, it is 

 obvious that the efificiency of the creature as a boring machine would, at once, 

 be much increased. The more effective the machine the less vital energy and 

 time would be required for its work, and natural selection would at once begin 

 to promote the development of the apparatus in the line indicated. It is prob- 

 able that the dorsal progress of the muscle is initiated by the wide space, or 

 gape, required for the foot to perform its functions in GastrochcBiia, which re- 

 duces the space available for the anterior adductor to a narrow triangle close 

 to the dorsal margin. The reflection of the dorsal valve-margin, visible in 

 Gastrochcena, has a use ; it renders the valves when wide open and fixed 



