80 INLAND FISHERIES. 



inches iu leugtli will cover themselves gradually in the course of 

 from'half to three-quarters of an hour, but they reach the usual 

 depth of several inches only after a much longer period. Very 

 large clams out of their burrows seem to be entirely helpless. In 

 the young, the foot is relatively very much larger than in the 

 adult, and extends from the anterior side of the visceral mass, 

 just under the mouth, far back on its ventral side. This condition 

 of the foot is almost exactly like that to be found in such a clam 

 as the quahaug ( Yenus riierceiiaria) in its mature state. 2fya has 

 probably descended from an ancestral form which possessed this 

 plowshare-shaped foot, its organ of locomotion being reduced to 

 its present form because it became less and less an organ of loco- 

 motion, and was used simply for digging downward into the sand. 

 We have a confirmation of this view in the structural peculiarities 

 of the foot in the very small 2fya, as described above. 



In the young 2£ya the foot is capable of great extension, and is 

 used not only in crawling over objects, but also in digging into 

 sand and mud. It is extremely interesting to notice that indi- 

 viduals but 1^ millimeters long (I have not happened to observe 

 it in smaller forms), when placed upon sand, at once attempt to 

 cover themselves by thrusting and worming the sharp anterior 

 part of the foot into it. Unless the sand be extremely fine, clams 

 of this size are not able to thrust aside the grains sufficiently to 

 obtain a lodgment. Those measuring 2 or 3 millimeters in length 

 are sometimes able to cover themselves partially or wholly ; while 

 an individual 6 millimeters long can usually work its way beneath 

 the surface of any clam bed, and thus rest in comparative security. 

 All clams which I have observed, under 6 or 8 millimeters in 

 length, work their way downward only far enough to cover the 

 shell. None of them seem to be overly energetic, and many times, 

 after working long enough to raise the posterior end of the shell 

 into a vertical position, they give up the attempt to bury them- 

 selves, and remain in that attitude until toppled over by the water 

 currents. After having become completely covered they exhibit 

 a great deal of restlessness, and apparently often push out to the 



