502 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
it to be readily recognized, and the writer follows the course already 
adopted by Verrill and others of rejecting Agassiz's name and adopt- 
ing that of Stimpson. 
Externally this species is commonly of very regular and symmetrical 
form, having an approximately circular outline when seen from one 
Text- fig. 15. — Caesira arenata (Stimpson). X 3. 
side except that between the apertures the outline is nearly straight, 
and the body is strongly flattened from side to side, often to such an 
extent as to be disk-like. The apertures are widely separated, their 
distance apart usually about equaling one fifth of the circumference 
of the body, and in the contracted alcoholic specimens they often 
project but little, sometimes scarcely at all, beyond the general outline 
of the body. Generally the body is smooth and evenl}' covered with a 
close coat of sand grains, which extends to the lobes surrounding the 
apertures. Occasionally shell fragments, minute pebbles, etc., also 
adhere to the body. The branchial aperture has as usual six lobes, 
the atrial four. This species does not appear to attach itself, but to 
live free, buried in the sand. Occasionally two individuals adhere 
loosely together. The largest specimens in the collections measure 
18.5 mm. to 20 mm. in length and scarcely less in depth, but often 
not morp than 7 mm. in transverse diameter. 
Test transparent or translucent, grayish or brownish in alcoholic 
specimens when the sand is removed, and moderately thick and tough 
in large specimens. When removed from the test, the body, owing 
to the widely separated and very short siphons has a more or less 
triangular outline with the apertures at two of the angles. The 
mantle is quite transparent, allowing the viscera to be clearly seen. 
On the sides of the body the musculature of the mantle is for the most 
part diffuse and very slight. The bands which radiate from the 
