562 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
waterworn sand projecting from the test. These minute projections 
are additional to (and often borne upon) the coarser wrinkles and 
rounded elevations of the surface of the test which are present in this 
as in most related species to a varying degree in different individuals. 
Inner surface of test smooth and more or less whitish or bluish, at 
least in alcoholic specimens. Size of largest specimens 18 mm. to 
20 mm. by 13 mm. to 15 mm. across the upper surface, but only from 
5 mm. to 9 mm. high (dorso-ventrally) in the contracted preserved 
specimens. 
Mantle musculature moderately developed on the dorsal and very 
weak on the ventral part of bod}'. Irregular and imperfect bands 
radiating from the apertures form a deep layer and are overlaid by a 
more or less continuous sheet of circular fibers. 
Tentacles exceeding fifty in number in large specimens, of four or 
five sizes (the smallest mere tubercles) arranged with some regularity. 
Dorsal lamina plain-edged and rather wide. 
Dorsal tubercle horseshoe-shaped with the open interval turned 
toward the left or somewhat forward in the individuals studied. 
Branchial sac with four well developed folds. Transverse vessels 
not very numerous, of three sizes in the dorsal, and four in the ventral 
part of the sac, the vessels of the fourth order merely crossing the stig- 
mata. Internal longitudinal ^'essels placed quite close together on 
the folds and usually separated only by the width of two, three, or 
four stiginata, even on the interspaces. They are much more numer- 
ous in large individuals than in small ones though separated by about 
the same number of stigmata. In a rather large specimen they are 
distributed thus: 
mdr. 3 (14) 4 (9) 5 (12) 4 (6) 3 en. 
In a still larger specimen growing attached to the same shell the num- 
ber is still greater: 
mdv. 4 (22) 6 (14) 5 (18) 6 (10) 8 en. 
but such large numbers do not occur in most specimens. 
Stomach rather broad, its pyloric end plainly differentiated from 
the commencement of the intestine; its wall with 20 or more longitudi- 
nal folds arranged in the usual manner. Intestinal loop fairly large, 
but the parts of the intestine become somewhat crowded together 
and displaced from their usual relations by the flattening of the body. 
