RETORT ON THE ASTEROIDEA. 101 



which increase in length and robustness as they approach the inner end of the mouth- 

 plates. 



The actinal interradial areas are large and well defined, and occupied by plates arranged 

 in regular series or columns passing from the adambulacral plates to the marginal plates ; 

 four or five plates may be counted in the series on each side of the median interradial 

 line, and intermediate plates extend as far as the fourth or fifth infero-margiual plate 

 and the eighth adambulacral plate. The surface of the intermediate or ventral plates is 

 covered with small, uniform, papilliform granules, forming more or less well-defined 

 rectangular groups. 



The madreporiform body is obscure and hidden by paxillse. It is near, but not adjacent 

 to, the marginal plates. 



The tube-feet are conical, and have a very small but distinct terminal mamelon-like 

 knob. 



Colour in alcohol, an ashy white, with a slight ochraceous shade on the paxillar area. 



Locality. — Station 78. Between San Miguel and St. Maria, Azores. July 10, 1873. 

 Lat. 37° 26' 0" N., long. 25° 13' 0" W. Depth 1000 fathoms. Volcanic mud. Surface 

 temperature 7l°"0 Fahr. 



Remarks. — This species is most nearly related to Plutonaster notatus, but is distin- 

 guished by the shorter, broader, and more robust rays. The marginal plates have also a 

 more massive appearance. The differences in point of structure are slight but note- 

 worthy, and the two forms are unquestionably closely allied. 



Subgenus Tethyaster, nov. 



The species placed in this subgenus differ from the other species of Plutonaster 

 chiefly in the character of the armature of the adambulacral plates. The whole armature 

 of the plate is spiniform, and is arranged in a co-ordinated group, the general plan of the 

 arrangement having a more or less striking resemblance to that found in Astropecten. 

 The adambulacral plates are much broader than in the true Plutonaster, and the madre- 

 poriform body is simple and exposed. 



This subgenus has been established for the reception of the two comparatively little 

 known but well marked species originally described under the names of Asterias subiner- 

 mis, Philippi, and Astropecten 'parelii, Ditben and Koren. The former of these was 

 ranked by Miiller and Troschel as an Astropecten, and subsequent writers followed this 

 reference until Perrier has recently placed the form in the genus Archaster. The second 

 species was referred to the genus Archaster by Sars, and that determination has been 

 maintained by succeeding writers. 



These two species are in my opinion very closely allied to the true Plutonaster species; 

 but as they exhibit the above-mentioned constant differences in their morphological 



