from Japan and the Indian Ocean. 85 



narrow, generally with a series of holes in the median line of 



tlie cover-plate. 



Splia3ridia3 are found on both sides, wanting only on the 



Iwo or three plates nearest to the apical area. Tliey do not 



present peculiar features. 



The spicules (PI. V. fig. 11) of tlie actinal tubs-feet are 



rather large, irregular, thorny, fenestrated plates, arranged in 



iour longitudinal series. The sucking-disk is well developed. 



Ilie abactinal tube-feet are large and prominent, with neither 



sucking-disk nor spicules. 



The test is of a faint flesh-colour ("incarnatus") *, in tvvo- 



of the specimens rather pale. The spines of the abactinal 



side are greenish, those of the actinal side whitish ; they aie 

 not ringed. 



In my work on tlie 'Ingolf^ Echinoidea I stated f that 



the tetradactjlous pedicellariai must probably be derived from 

 the tridentate ones; de Meijere (' Siboga ^ Echinoidea, p. o5) 

 thinks it more probable tliat they are derived from the 

 ophicephalous form. It does not seem to me very probable 

 that they be developed from either of these forms. There is, 

 indeed, nothing in the structure of the tetradactylous pedi- 

 cellarije which points in eitiier direction, and I do not see 

 why they cannot represent a special form of pedicellaria?, 

 developed independently of the other forms. By their 

 glands (such are evidently also found in the most developed 

 forms, those of A.fenestratum a\}d coi'iaceuni) they remind us 

 of the globiferous pedicellariaj in other regular Echinids, and 

 they n)ust certainly be regarded as analogous, if not homolo- 

 gous, with the globiferous pedicellarije. The form of pedi- 

 cellaria in Hapalosoma described by me in the 'Ingolt* 

 Echinoidea (p. 55) as a primitive globiferous pedicellaria, 

 not seeing its relation to the tetradactylous pedicellaria;, 

 cannot, of course, any longer be regarded as a primitive form, 

 now that the three-valved "tetradactylous^' pedicellarige of 

 A. tessellatum and A. Owsteni have been made known. As 

 rightly pointed out by de Meijere, it must be regarded as a 

 case of extreme development, in which the valves have become 

 rudimentary and the glands excessively developed. 



The form of these curious tetradactylous pedicellarife shows 

 such gradual changes that it seems reasonable to regard 

 A. Owsteni as the most primitive of the species of Arceosoma ; 

 the fact that the largest form of tridentate psdicellarias is 



* Saccardo, ' Chromotaxia,' ii. ed. (1894). 



t Accordinj^ to de Meijere. I am unable to find the place where the 

 etatemeut occurs, but I dare not deny that I really made it. 



