REPORT ON THE ASTEROIDEA., 81 
plates appear to form too broad a border on the abactinal surface for this genus; but we 
find that in the young stages of other species of this genus (e.g., in Dytaster biserialis and 
Dytaster exilis) the supero-marginal plates form a definite border in the immature phase, 
even when they are confined entirely to the lateral wall in the adult. In like manner the 
very small and limited actinal interradial areas seemed at first to throw doubt upon the 
correctness of the reference of this example to Dytaster, but this again is found to be war- 
ranted by the early condition of other species. The character of the adambulacral plates 
and their armature, the character of the abactinal paxille and of the pedicellariz, accord 
perfectly with those of Dytaster. On these grounds I have placed it in this genus, and 
consider that it is probably nearly allied to Dytaster biserialis. 
The form, as we know it at present, is readily distinguished from all the other members 
of the genus by the unarmed marginal plates, the character of the abactinal paxille, and 
the simple armature of the adambulacral plates. 
Genus Plutonaster, Sladen. 
Plutonaster, Sladen in Narr. Chall. Exp., 1885, vol. i. p. 610. 
Disk comparatively large and flat. Rays usually elongate, more or less rigid. 
Marginal plates rectangular ; the supero-marginal plates thick and massive, forming a 
well-defined and often broad border on the abactinal surface. The plates of the superior 
and inferior series directly superposed, contingent horizontal margins straight. The supero- 
marginal plates usually devoid of spines; the infero-marginal plates frequently with one 
small rudimentary spine, but even this is sometimes aborted or absent altogether. The 
general surface of the plates of both series covered with small papilliform granules. 
Abactinal area with small closely packed pseudo-paxillz. No definite medio-radial line ° 
of plates. Papulz generally distributed. Abactinal plates at the sides of the ray arranged 
in more or less definite obliquely transverse series. 
Actinal interradial areas large, with numerous well-defined intermediate (ventral) 
plates, arranged in regular columns, the breadth of the plates decreasing as they approach 
the margin. 
Armature of the adambulacral plates consisting of :—(1.) A longitudinal furrow series 
of short, subequal, cylindrical spinelets, sometimes radiating slightly apart. (2.) Two or 
more longitudinal series of papilliform granules on the actinal surface, the innermost occa- 
sionally spiniform. One large conical spinelet may be present on the actinal surface, but 
frequently only on plates near the extremity of the ray. In some forms (the subgenus 
Tethyaster) the armature of the actinal surface of the adambulacral plates is disposed in a 
co-ordinated group rather than in definite longitudinal series, and in these cases it is 
usually more distinctly spiniform in character. 
Madreporiform body large, sometimes compound, placed about its own diameter distant 
from the margin, more or less concealed by paxillee (except in the subgenus Zethyaster). 
(ZOOL, CHALL. EXP,—PART LI.—1887.) 11 
