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 1 

 of certain specimens which he has studied and referred to Gray's species. There is, how- 

 ever, no type of Acanthaster ellisii 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 



O 



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 pedicellarise 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 

 ellisii. 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 Mithrodiin^e, 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. Notwithstanding 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 Revis. Stell. Mus., p. 99 {Archives de Zool. ezper., 1875, t. iv. p. 363). 



