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spine, near the base of the arm, may be distinctly longer, thinner 
and more smooth than the other, but this is no constant character. 
The three lower spines are equal sized, about the length of the 
arm joint. They are finely serrate and slightly bihamulate. Mouth- 
papillæ two, sometimes only one, on each side of mouth angle, 
the outer one the larger; they are generally erect. Adoral plates 
large, mouth shield almost rhomboidal, as long as broad, with rounded- 
truncate outer angle. — There is sometimes a trace of redbrown 
color at the base of the spines in fullgrown arms. 
The species is selfdividing; all stages of reproduction of half 
the disk with its three arms are met with. The faet that three 
and three arms are of the same size means that, in case self- 
division takes place more than once, the division line remains the 
same. One exception from the rule was seen, however, a recently 
divided specimen having three arms of different length, one of 
them belonging distinctly to the reproduced, not yet fully grown 
half; in this case accordingly this second division took place after 
another line than the first. 
Several specimens were found by the author in the Trondhjem- 
fjord, in the following localities: Skarnsund, ca. 200 meters, off 
Tautra, ca. 200 m, and off Rødberg, ca. 300 m, in July 1911. 
Further, some specimens from Hellefjord, 200 fms, taken by the 
Norwegian North Atlantic Expedition 1876—78, wrongly identified 
as O. abyssicola, are really this species. Doubtless the species will 
be found to be widely distributed over the North Atlantic. 
Evidently this species is related to Ophiactis hirta Lyman. 
(„Challenger“ Oph., p. 118, PI. XX, Figs. 4—6). It differs from 
that species, besides in the number of arms (7 in O. hirta , 6 in 
nidarosiemis), in the shape of the ventral plates, which are shorter 
in O. hirta , only as long as broad; also the mouth shield appears 
to be somewhat different in shape, and the mouth papillæ are 
smaller in O. hirta than in nidarosiensis. Finally the size of O. 
hirta was 4.2 mm in diameter of disk or nearly twice the size of 
the largest specimens of the present species. 
Judging from these differences there can hardly be any doubt 
that this species is not identical with O. hirta , as one would also 
beforehand be inelined to expect in view of the faet that O. hirta 
