ASTEROIDEA OF U.TiJ.S. ' CHALLENGER ' EXPEDITION. 243 



Benthaster wtville-thomsoni, u. sp. 



Marginal contour substellate ; rays broad at base, and tapering 

 to a verj'^ fine extremity ; interradial angle well indented, not 

 rounded ; the lesser radius in the proportion of 50 per cent. ; 

 ^=18 millim., r = 9 niillim. Greneral body-profile much depressd, 

 slightly elevated in the centre of the disk. 



The dorsal aspect is very remarkable, recalling at first sight 

 the aj)pearance of Korethr aster. This resemblance, however, is 

 merely illusory, and arises from the presence of prominent tufts 

 of long spinelets that project free beyond the dorsal membrane. 

 The pedicels of the paxillae are comparatively short, reduced almost 

 to tubercles on the outer part of the rays, bearing about 8 to 10 

 spinelets, which are of great length and expand very slightly apart 

 from one another. Paxillse standing on cruciform ossicula, the 

 prolongations of which are very long and thin, and the central 

 portion where they cross little, if at all, widened. 



Supradorsal membrane represented by a loose irregular spongi- 

 form tissue, which fills up the paxillse -crowns near their bases, and 

 extends over the whole dorsal area. This spongy mass is not 

 uniform in thickness or density, and nowhere forms a definite 

 membrane. The paxillse- spinelets protrude a great portion of 

 their length through this tissue, and have the appearance of being 

 entangled amongst it — a conventional definition of their character 

 expressing more than any rigid description of this part of the 

 structure. No muscular fibrous bands, and no spiracula present. 

 The spinelets are transparent and vitreous in appearance, regu- 

 larly trilaminate (which may be distinctly seen in every broken 

 section), and taper to a fine sharp point. No trace apparent of 

 any true membranous envelope to the paxillse. The spinelets on 

 the disk are much longer and more robust than elsewhere, 

 attaining their greatest size in the neighbourhood of the centre. 

 Oscular orifice rather indistinct, margined by five somewhat irre- 

 gular tufts of spinelets, longer and more robust than any of the 

 others. No definite or regular valves appear to be formed. It 

 is doubtful to what extent the dermal chamber is developed; but 

 probably its character is greatly modified : the specimen under 

 notice leads to the inference that it is almost aborted in the 

 present instance. 



At the extremity of the rays there is on either side an elon- 

 gate dorso-marginal plate, equal in length to about 7 or 8 

 segments of the ray, becoming thicker at the aboral end and 



