538 THE VOYAGE OF H.M.S. CHALLENGER. 
Localities.—Zebu, Philippine group. On the reefs. Depth and conditions not 
recorded. 
Kandavu, Fiji Islands. On the reefs. Depth and conditions not recorded. 
Remarks.—These examples appear to me to accord with the description given by Perrier* 
of certain specimens which he has studied and referred to Gray’s species. There is, how- 
ever, no type of Acanthaster ellis in the British Museum; it is therefore extremely 
doubtful to what form that author applied the name. In the examples under notice the 
spines are all covered with a thick, closely fitting, fleshy, whitish membrane, which causes 
them to appear perfectly smooth and glistening in the specimens preserved in spirit, and 
when the membrane is removed the spine is found to be perfectly smooth. When a spine is 
dried, however, it appears covered with regularly disposed granules which produce a rough- 
ness similar to that assigned as a characteristic feature in Acanthaster echinites. The 
granules, however, are much less pronounced, and are devoid of the central hair-like needle 
often found in well-preserved examples of Acanthaster echinites. The “roughness” of the 
spines in the Challenger examples is undoubtedly produced by the drying out of granular 
deposits contained in the investing membranous sheath, and I am inclined to believe from 
the examination of dried specimens of Acanthaster echinites that the same explanation 
will hold good for that form. 
The pedicellarize in the examples from Zebu and Kandavu appear shorter than in 
specimens from Mauritius which have been referred to Acanthaster echinites, a circum- 
stance also mentioned by Perrier as a character of the form he refers to Acanthaster 
ellisiz. The colour is a bright whitish violet. 
Notwithstanding the differences noted above, I feel grave doubt as to whether they 
are of sufficient importance to warrant the specific separation of the form from Acanthaster 
echinites; but with the material at present available I am not in a position to fully 
discuss the question. 
Subfamily Mirnropuna@, Viguier, 1878. 
Genus Mithrodia, Gray. 
Mithrodia, Gray, Ann. and Mag. Nat. Hist., 1840, vol. vi. p. 287. 
Heresaster, Michelin, Revue Zoologique, 1844, p. 173. 
This well-marked form, which has given so much trouble to classifiers, is essentially 
an inhabitant of tropical seas. N otwithstanding its wide area of geographical distribution, 
the specific character is maintained with great constancy, and the genus shows a very 
limited range of morphological plasticity, three species only having been defined. 
1 Révis. Stell. Mus., p. 99 (Archives de Zool. expér., 1875, t. iv. p. 363). 
