438 



THE AMERICAN NATURALIST 



[Vol. LIV 



sometimes forms, in addition, a more or less projecting "shoul- 

 der" (Fig. 4). These departures from the smoothly circular 

 form, together with the "arched" construction of the test in 

 some older specimens (Fig. 4), derive their effectiveness for 



burrowing from the partial rotation or "swinging" of the disk 

 during this act. 



The changes here noted in the form of the body of some indi- 

 viduals with advancing growth, are not detectably correlated 

 with peculiarities of habitat. The different types occur with 

 about the same relative frequency whether the bottom is of shell 

 sand or of brownish mud. The local character of the bottom 



