SOME CASES OF ABNORMAL ARM STRUCTURE IN 
RECENT CRINOIDS. 
By Austin Hoparr Criark, 
Of the United States Bureau of Fisheries. 
Several cases of abnormal arm structure among the recent crinoids, 
especially the comatulids, have been recorded by Dendy, Bateson, 
W. B. and P. H. Carpenter, Chadwick, and Sars; and it is the object 
of the present paper to put on record some additional cases which 
have recently come under the author’s observation. 
Rhizocrinus lofotensis M. Sars. Although Sars has shown that 
this species in the Lofoten Islands varies in the number of its arms 
from four to seven (recording, out of a total of seventy-five speci- 
mens, thirty-two with four, six, and seven arms, and forty-three with 
the normal five) those recorded from the American side of the Atlan- 
tic have all been five-armed. I have a specimen before me, however, 
from Albatross Station No. 2666, 30° 47’ 30’’ north latitude, 79° 
49’ 00’’ west longitude (between the Bahamas and Cape Fear, North 
Carolina), 270 fathoms, which has six arms, showing that this varia- 
tion, though comparatively rare, does occur on the American coast. 
Pecilometra acala (P. H. Carpenter). The only specimen of this 
species which was dredged by the Albatross off southern Japan has 
a third costal inserted between the normal two on one 
of the rays (fig. 1), as was the case in a specimen of 
Thaumatometra alternata dredged by the Challenger 
just north of New Guinea. The radials in this speci- 
men are entirely hidden by the centro-dorsal. Mr. 
Frank Springer, in his monograph on Uintacrinus, fig. “Ve TO 
ures a specimen of U. socialis (Plate m1, fig. 2) with costar series 
four costals on one ray, an additional pair being in- | yy rone 
serted between the normal costals and the first bra- 
chials; from the shape and proportions of these additional costals 
in this specimen one might almost infer that they were united by 
syzygy, in which case the costal series would be directly comparable 
to the distichal series in comatulids with 4 (8 -+ 4) distichals; the 
PROCEEDINGS U.S. NATIONAL Museum, VOL. XXXIV—No. 1612. 
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