I3O OILMAN A. DREW. 



doubt that specimens are easily disturbed by vibrations of the 

 bottom. That they habitually burrow to some depth is indicated 

 by the fact that after the first half dozen trials, specimens are not 

 usually obtained without going to another spot several feet away, 

 although the first trials may have resulted in one or more speci- 

 mens each. 



When dug from the mud, individuals frequently leap or swim. 

 In leaping the shell may be thrown several inches by the action 

 of the foot. In swimming the animal progresses posterior end 

 first and large specimens may swim several feet before stopping. 

 The foot is always active while the animal is swimming. These 

 activities, together with the movements of burrowing, will receive 

 attention later. 



Before describing the movements it is desirable to call atten- 

 tion to some points of anatomy. 



The shell is of nearly even diameter both dorso-ventrally and 

 laterally (Figs. I and 4), throughout its length, except very near 

 its ends. Anteriorly it contracts in both directions a little, but 

 the anterior margins of the shell valves remain wide apart even 

 when they are in contact along their ventral borders (Fig. 6). 

 This leaves ample space for the protrusion or withdrawal of the 

 foot when the shell is closed. The anterior margins of the lobes 

 of the mantle are thickened and extended, so, when the foot is 

 withdrawn into the shell, these flaps cover the opening between 

 the shell valves (Fig. 6). When the foot is extended the flaps 

 are spread apart and form a collar around the foot, the free 

 margins of which are in contact with the foot (Figs. I and 4). 

 The collar is thick and muscular, being well supplied with the 

 radial pallial muscles, and as it is held tightly against the foot, 

 forms a very effective scraper, that cleans the foot so mud is not 

 drawn into the shell with it. The cilia covering the foot no 

 doubt aid in loosing the mud so it is easily scraped off. The 

 flexible collar adapts itself to the shape of the foot so its margin 

 is applied to the surface of the foot until its very extremity is 

 drawn into the shell (Figs. 6 and 7). 



The posterior end of the shell narrows laterally, but here again 

 the margins of the shell valves are wide apart when the ventral 

 edges of the valves are in contact (Fig. 5). This makes it pos- 

 sible for the siphons to be at least partially extended when the 



