REPORT ON THE ASTEROIDEA. 121 



Infero-marginal plates extending on the actinal area. Supero-marginal plates crowded 

 with numerous small, uniform, cylindrical, miliary spinelets ; no large prominent spines. 

 Infero-marginal plates covered with large, flat, squamiform, adpressed spinelets, with one 

 or more similar, large but short, flattened spinelets at the margin adjacent to the supero- 

 marginal plates. 



Abactinal area with subhexagonal oblong imbricating plates, bearing paxilliform 

 groups of short spines. A conspicuous medio-radial series larger than the rest. The 

 other plates form regular obliquely transverse series, each plate imbricating on the next 

 in its own series by a single prolongation of peculiar form developed from the inferior 

 surface of the plate. Papulae regularly distributed. 



Actinal interradial areas almost nil, with very few actinal intermediate plates. 

 Armature of the adambulacral plates triserial, simulating that of Astropecten. A 

 series of three geniculated pointed spines in triangle on the furrow margin ; followed by 

 one or two outer series of two to four flattened spines. 



Madreporiform body in mid area. Occasional pedicellarias (subforficiform) are present 

 in the median series of spines on the actinal surface of the adambulacral plates. 



Remarks. — The genus Archaster, as originally constituted, comprised the two species 

 named by its founders Archaster typicus and Archaster hesperus ; both are very remarkable 

 forms and are widely separated from one another structurally. Indeed it is impossible any 

 longer to retain them in the same genus ; and it is difficult to account for their long 

 companionship except on the ground that Archaster hesperus is of rare occurrence and 

 has seldom been brought to Europe, and that nearly all the specimens preserved in 

 museums are dry and more or less damaged. The only point then to determine is as to 

 which of the two forms should stand as the type of the genus Archaster, which, in other 

 words, would have been so regarded by its founders, and which of the two represents best 

 the characters mentioned in their brief and very general diagnosis of the genus. That 

 Archaster typicus fulfils these conditions I think there can be but little doubt ; in the first 

 place there is the specific or trivial name ; 2d, its priority in the order of description ; 3d, 

 its close conformity with the generic diagnosis, which would be very insufficient for a well- 

 preserved example of Archaster hesperus; 4th, I am extremely doubtful whether any anus 

 is present in Archaster hesperus, and the presence of this aperture was in Midler and 

 Troschel's r opinion [the most marked character of all. On these grounds I consider 

 Archaster typicus as the type form of the genus Archaster ; whilst Archaster hesperus 

 constitutes the type of a genus for which I propose the name of Craspidaster, the 

 characters of which will be discussed on a subsequent page. 



Of all the species which have been referred to the genus Archaster since 1840 one 

 only — Archaster angulatus of Miiller and Troschel — presents the same structural characters 

 as the type ; the other forms have now been distributed amongst several different 

 genera. 



(ZOOL. CHALL. EXP. — PART LI. — 1887.) 16 



