ASTEBOIDEA OF H.II.S. ' CHALLENaER ' EXPEDITION". 203 



with the mouth-spine series by a continuation of the web. Mouth-spines 

 three on each plate^ webbed together. Actino-lateral spines merged in 

 the actinal floor ; no independent marginal fringe. 



This genus is nearly related to Pteraster, — from which, liow- 

 ever, it is distinguished by the rudimentary character of the 

 supradorsal membrane, devoid of muscular fibrous bands ; by tlie 

 actino-lateral spines being merged in the actinal floor ; and by 

 the absence of a marginal fringe. Furthermore, the xaumerous 

 hair-like spinelets of the paxillse protruding through tlie mem- 

 brane give a peculiar character to the dorsal area ; and the am- 

 bulacral spines, from their high position in the furrow and from 

 their more or less horizontal direction, present a feature unknown 

 in the other members of the group. 



Maesipaster spinosissimus, n. sp. 



Marginal contour pentagonoid ; radii somewhat produced ; in- 

 terradial angles widely rounded, forming a continuous curve from 

 tip to tip. Lesser radius in the proportion of 62*5 per cent. ; 

 i2=16millim.. r=10 millim. Greneral form depressed; dorsal 

 area slightly convex, tapering off gently to the extremity of the 

 rays. 



Supradorsal membrane very fine and thin, somewhat irregular, 

 and forming a continuous spongiose mass, rather than a definite 

 membranous sheet, through which the spinelets of the paxillfe 

 protrude freely. No specialized muscular fibrous bands present. 

 Spiracula very few, widely and irregularly spaced. 



The paxillse have long pedicels and a crown consisting of a 

 great number (20-30 at least upon the disk) of very fine hair- 

 like spinelets, which radiate at a small angle from the perpen- 

 dicular, crown and pedicel alike being invested with a delicate 

 membranous tissue, the whole appendage seen in profile bearing 

 a fanciful resemblance to an umbrella when turned inside out. 

 The investing membrane merges into the general spongy tissue 

 above mentioned ; and a considerable length of the extremities of 

 the spinelets protrudes free and naked. The paxillse are nume- 

 rous ; and their crowns join up close together. Owing to these 

 circumstances and to the great number of the spinelets, the 

 dorsal surface presents the appearance of a coarse, irregular 

 velvet pile. Oscular orifice small and quite inconspicuous ; 

 valves consisting of a flattened crown of rather more robust 

 spinelets. 



