380 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
.should be adopted as the specific name. Verrill considered white 
colonies of this animal as constituting one species {albidum) and 
pinkish or yellowish specimens another species or variety (liiteolum). 
Apparently the usual color of colonies of this species in European 
waters is rosy, though a white variety occurs also. In American 
waters, white specimens seem to be more common. 
The following description is based exclusively on American speci- 
mens : 
The colonies are of a white, rosy, salmon or yellowish color, generally 
only 2 mm. to 3 mm. in thickness and of irregular and variable outline, 
occasionally measuring over 100 mm. across but usually not so much. 
They incrust stones, shells, sponges, etc., and not infrequently the 
larger simple ascidians. In consistency, they are generally rather 
hard and brittle, owing to the great abundance of large spicules. 
Text-fig. 13. — Tetradidemnum albidum (Verrill). Typical spicules from a colony from 
off Nova Scotia. X 340. 
These are in most colonies rather thickly distributed throughout the 
test, but most densely crowded in the upper layers. They render the 
surface slightly rough and gritty to the touch. The cloacal openings 
are usually inconspicuous in preserved and contracted examples, and 
the branchial apertures are also generally not at all prominent. Occa- 
sionally their position is marked by small circular areas where the 
spicules are less abundant; more frequently the surface of the colony 
is raised into small low rounded elevations which mark the positions 
of the zooids. Usually the surface is not greatly wrinkled. 
The spicules (text-fig. 13) are very characteristic and quite unlike 
those of typical members of the genus Didemnum [Leptoclinum]. 
They are of large size, averaging perhaps O.OS mm. in diameter in 
many specimens, but in others, especially in those from more southern 
localities, not more than 0.05 mm. The colonies with the largest 
