322 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
old, that the difference between them could not be told at a glance. 
Their only resemblance is in the color markings which in rare instances 
are alike. 
In studying alveus alive it is found to move freely on the eel grass, 
swinging its head from side to side, its tentacles projecting far beyond 
the lateral edge of the shell; tcstudinalis, on the contrary remains fixed 
for hours and only in the extreme young have I seen considerable 
freedom of motion. INIr. Dwight Blaney has found alveus on the 
under side of stones on a coarse pebbly beach at Ironbound Island, 
Maine, and I have observed it in similar situations in Salem Harbor, 
Massachusetts. In both these instances, however, the creature might 
have become detached from its usual resting place on eel grass by storms 
and washed by the waves to these unaccustomed places. 
In the young tesiudinalis a millimeter or more in length, the shell is 
rounder, the apex more elevated, blunter, and brown in color; the 
radiating lines being scarcely visible. In alveus of the same size the 
shell is longer, apex not so elevated, white in color and the radiating 
lines are sharply defined. 
I am greatly indebted to Dr. Charles G. Weld and Dr. Harold S. 
Colton for important aid in collecting material for this paper. 
