188-J.] SPECIES OF OREASTER. 79 



Sides of the arms formed by the superomarginal plates only: of these 

 there are rather more than 20 on either side of each arm ; they are 

 regularly provided with long strong spines, which are nearly always 

 rather acute, and only become distinctly shorter near the distal end 

 of the arm. The inferomarginal plates are not quite so numerous as 

 the superomarginal ; they are almost without exception provided with 

 cue, and in some cases with more than one, spine ; these vary a little 

 in size, but are always much smaller than those on the upper plates. 



Adambulacral spinulatiou diplacanthid, three inner and one outer 

 being the ordinary arrangement ; the inner are quite small, the outer 

 of fair size, and not sharp at their tips. The whole of the ventral 

 surface is spinous, owing to the rich development and the regular 

 distribution of the spines which are found on it ; there are as many 

 as six rows in the proximal and three in the distal region ; the spines 

 would seem to be carried on separate ossicles, and while each ossicle 

 has one, it may sometimes have two or three large spines, or a 

 larger number of smaller ones ; speaking generally, the larger spines 

 are the more proximal. Between the spines are found coarse, sepa- 

 rate granules, and a fair number of sessile bivalved pedicellariae. 



In the specimen under description the bases of the ossicles are 

 particularly thick, except along the lophial line ; a large number of 

 the pore-areas have the form of an equilateral triangle, and are quite 

 distinct from one another ; as a rule the best-developed spines of the 

 dorsal surface are those which are found on the nodes of the reticu- 

 lations ; between these somewhat smaller ones are not unfrequently 

 developed; the spines along the lophial line are, on the whole, 

 rather stouter than the rest, but they are by no means conspicuously 

 so. There are no prominent spines on the apical portion of the disk ; 

 and the madreporite is of moderate size and somewhat irregular 

 shape. 



Colour dark brown ; but other specimens are white. 



Measurements: — R = 186; »- = 86; height of disk 116 millim. 



Hab. "West Indies;" two small specimens in the Museum 

 collection are from Fremantle, Australia. 



Owing, no doubt, to the great size to which this species attains, 

 many of the incompletely grown forms have been regarded as repre- 

 sentatives of distinct species ; nor has this, as it seems, been the 

 only cause of the many names given to it. The O. lapidarius of 

 Grube is no doubt a remarkable form at first sight, but the ap- 

 pearance is to be explained by the dried and injured condition of the 

 specimen. 



Oreaster armatus. 



Pentaceros {NidoreUia) armata. Gray, Ann. N. H. vi. 



Oreaster armatus, M. Tr. Syst. Ast. p. 52. 



NidoreUia armata, Perrier, Rev. Stell. p. 251. 



NidoreUia michelini, Perrier, Rev. Stell. p. 252. 



The conception of the limits of a genus vary, of course, con- 

 siderably, but the close study of this species seems to show that 

 M. Perrier has elevated into generic what are only specific cha- 



