VANxXAME: COMPOUND ASCIDIANS. 415 
England. The presence of this northern Tetradidemnum, readily 
and positively identified even in poor preservation by its characteristic 
spicules, certainly tends to confirm the opinion that the Amaroiiciuni 
colonies found growing with it are really A. glahruin. \'errill (1875j 
reports A. glabnim also from oft" Block Island, R. I. 
The greatest depth from which any of the specimens that the writer 
has examined, came is Station 2699 (off Newfoundland, X. lat. 45° 04', 
W. long. 55° 23', 72 fathoms, August 22, 1886). 
Ritter (1901) has described a species (^4. traushieidum) evidently 
very closel}' allied to the present one from numerous specimens col- 
lected on reefs at low tide in Prince William Soimd, Alaska. In some 
respects his specimens differ from .4. glabrum as it is ordinarily found 
on the Atlantic coast, notably in the light red color suft'using the test 
and zooids, the very regular inversely conical colonies, the larger and 
more crowded zooids, the very transparent test, and the usually larger 
number of folds in the stomach wall, which folds, judging from his 
statements, are more subject to irregularity in arrangement than in 
the Xew England form. Though his sj)ecimens were all from one 
locality, thus increasing the probability that the above discrepancies 
may be only manifestations of individual variation or the ett'ect of 
some ])articular environment, yet the writer would be disposed to 
regard Ritter's form as an allied but distinct species were it not that 
Hartmeyer (1903) and Redikorzew (1906, 1907a, b, 1908b) have 
identified with Ritter's species, specimens from Spitzbergen and Bear 
Island, the Murman coast, the White Sea, and Siberian waters, with 
characters apparently completely bridging over the differences between 
the Alaskan and New England forms, and indicating that we probably 
have to deal with a single species of circumpolar distribution, which, 
however, may be somewhat differentiated into geographical races in the 
Atlantic and Pacific regions. Should this view be accepted, glabnim 
would be the specific name, while franslucidum might be employed as 
a trinomial to designate the Alaskan race or subspecies. The Euro- 
pean specimens should, as far as can be judged from the descriptions 
and figures, be regarded as of the typical glabrum form rather than of 
the iranslucidum form. 
