484 THE VOYAGE OF H.M.S. CHALLENGER. 



Supradorsal membrane an irregularly developed, somewhat spongiform, tissue. No 

 muscular fibrous bands. No spicules. Paxillse with moderately expanded crowns com- 

 posed of a great number of fine, long, hair-like spinelets (fifteen to thirty), their extremities 

 protruding freely through the membrane. Paxillse invested with a well-developed mem- 

 branous envelope. 



Adambulacral armature forming transverse combs, consisting of spines webbed 

 together ; the spinelets more or less horizontal in position, overspanning the furrow. 



Mouth-plates with one secondary or superficial spine on the actinal surface, connected 

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

 each plate, webbed together. 



Actino-lateral spines merged in the actinal floor ; no independent marginal fringe. 



Remarks. — This genus is nearly related to Pteraster, — from which, however, it is dis- 

 tinguished by the rudimentary character of the supradorsal membrane, devoid of muscular 

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

 absence of a marginal fringe. Furthermore, the numerous hair-like spinelets of the paxillse 

 protruding through the supradorsal membrane give a peculiar character to the abactinal 

 area ; and the spines which constitute the armature of the adambulacral plates, 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. 



The representatives of this genus collected by the Challenger are all from the Pacific ; 

 the occurrence of an Atlantic species is, however, reported by Perrier ' in a summary of 

 the novelties obtained by the " Talisman " Expedition, but no description has yet been 

 published, and the position of the dredging station is not stated. 



Chorology of the Genus Marsipaster. 



a. Geographical distribution : — 



Atlantic : One species between the parallels of 10° and 50° N. 



Marsipaster alveolatus, dredged by the " Talisman " Expedition. 



Pacific : Two species between the parallels of 30° and 40° S. 



Marsipaster spinosissimus, in the Mid-South Pacific, near the 

 meridian of 135° W. Marsipaster hirsutus, between Valparaiso 

 and the Island of Juan Fernandez. 



/3. Bathy metrical range : 2160 to 2335 fathoms. 



-y. Nature of the Sea-bottom : Marsipaster spinosissimus on Red clay ; Marsipaster 

 hirsutus on Blue mud. 



1 Ann. Sci. Nat. (Zool.), 1885, 6e S&ie, t. xix. Art, No. 8, p. 69. 



