ANNOTATED LIST. 139 
Zoélogy is 18 mm. across the disk. The single individual collected at Mer was found on 
the underside of a rock-fragment on the southeastern reef-flat. It is not unlikely that 
further knowledge of the habits and morphology of this species will justify the recognition 
for it of Smith’s genus Acantharachna. 
OPHIARTHRUM. 
Peters. 1851. Monatsb. K.-Preuss. Akad. Wiss. Berlin, p. 463. 
Like Ophiomastiz, this genus is so easily recognized and the specific limits within it 
are so well defined, there are no synonyms and no wrongly assigned species to cause con- 
fusion. Only three species have been described, and each of these is valid. The genus 
may be regarded as a specialized offshoot of Ophiocoma, with a deficiently calcified disk, 
only 1 tentacle-scale and 3 or 4 arm-spines. It has a very wide range from Mozambique 
and Zanzibar to the Society Islands. It is not known, however, from the Hawaiian group 
as yet. The three species are not morphologically very different from each other, and the 
form of the plates of either arms or oral frame afford no very distinctive characters; but 
the coloration is an unfailing means of distinguishing the species, for while it is not by any 
means constant within narrow limits, each species is entirely different from the others. 
Key to the Species of Ophiarthrum. 
Arm spines light, spotted or ringed with dusky. 
Disk very dark, at least at center; no yellow; no median dark line on upper side of finely speckled arms. . elegans 
Disk beautifully marked in life with white, yellow! and gray; arms with a more or less distinet median 
darktinexon uppersidetsan ccs seter- ee istic ce ose nie’ t tein wi tancrade soi entnee pe\sseleacte cine pictum 
Arm-spines, and arms, uniformly grayish-brown; disk black. ..............:2 eee cece cece cece ee entero lymani 
Ophiarthrum elegans. 
Peters. 1851. Monatsb. K.-Preuss. Akad. Wiss. Berlin, p. 464—Koehler. 1898. Bull. Sci., 31, pl. iii, figs. 
25, 26. 
(Plate 13, Figure 1.) 
The centrally dark and peripherally light disk make this species easy to recognize, 
but there is great diversity in the colors of the living animals. Those found at Tahiti on 
August 5, 1913, had the disk deep chocolate-brown and the arms were banded with that 
shade and with deep brick-red; but these colors were much changed in alcohol. At Green 
Island, Queensland, where elegans is very common, none of the individuals showed any 
trace of red and the disk was dark brown with a purple tinge. At Mer the general colora- 
tion was very deep purple and greenish-white; there was great individual diversity in the 
amount of light color on the disk, but many had the arrangement of dark and light areas 
more or less as in the specimen figured (pl. 13, fig. 1). 
The distribution of this species is notable, as it is one of the few members of the 
family occurring in the neighborhood of Thursday Island. We found it common, but 
oddly enough it was not taken by the earlier collectors. Its known range extends from 
Mozambique and Zanzibar eastward to the Society Islands, and from Green Island, Queens- 
land, northward to the Riu Kiu Islands. It occurs under rocks and coral fragments on the 
reef-flats, like an Ophiocoma, and is moderately active when its shelter is disturbed. It is 
rather more delicate than an Ophiocoma and the surface of the disk seems to disintegrate 
or rub off very easily if the specimen remains long in a crowded collecting bucket. It was 
not easy at Mer to get specimens back to the laboratory in an uninjured condition. 
The largest specimen of elegans that I have seen is from Samoa. It is 23 mm. across 
the disk and is further notable for having siz arms, a very unusual freak. De Loriol (1893) 
says the arms are 9 times the disk-diameter, but in those which I have measured the pro- 
portion is 5.5 to 8 to 1. 
1In preserved specimens, which are often quite brown, the yellow shades fade and may disappear or become 
brown, and the white becomes dingy. 
