KErOKT 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 paxillse and of the pedicellarise, 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 paxilke, 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. Kays 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-paxillse. No definite medio-radial line 

 of plates. Papulse 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 

 Tetliyaster) 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 paxiLke (except in the subgenus Tetliyaster). 



(ZOOL. CHALL. EXP. PAET LI. 1887.) 11 



