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



Three pairs of secondary mouth-spines, free and inde- 

 pendent Calyptmtter. 



b. Armature of the adambulacral plates not forming transverse combs. 

 Spines independent and not united by web. 



a. Nidamental cavity spacious. Supradorsal membrane well-deve- 



loped. Muscular fibres present. Spiracula present. Spine- 

 lets of paxillae short, not protruding through, but supporting, 

 the membrane .......... Hymtnasttr. 



b. Nidamental cavity aborted. Supradorsal membrane rudimentary. 



No muscular fibres. No spiracula. Spinelets of paxilles- 

 fascicular, protruding a great portion of their length naked 

 through the membrane ........ Benthader. 



B. No supradorsal membrane present. No actino-lateral spines. No seg- 

 mental apertures. Form stellate, wys greatly produced, attenuate 

 nnd cylindrical. Abactinal spines shorr, in sheathed fascicules, not 

 forming pedicellate paxillae ........ Pytuonasteuisjj. 



a. A single genus ........... Pytlwnader. 



Subfamily Pterasterin^e, Sladen, 1888. 

 Genus Pteraster, Muller and Troschel. 

 [ Pteraster, Muller and Troschel, System der Asteriden, 1842, p. 128. 



With the exception of two Atlantic species, Pteraster caribbseus and Pteraster sor- 

 didus, all the members of this genus are confined to the colder temperate and frigid 

 zones. Notwithstanding its wide range of distribution the genus appears to show only 

 a comparatively small amount of morphological plasticity. 



Chorology of the Genus Pteraster. 



u. Geographical distribution : — 



Atlantic : Five species between the parallels of 80° N. and 60° S. 



^Pteraster militaris, off the Norwegian, British, and North 

 American coasts, extending up Smith's Sound as far as Dobbin Bay, 

 and south to Cape Cod, off Spitsbergen and Nova Zembla, and in the 

 Kara Sea. Pteraster pidvillus, off the Norwegian and North American 

 coasts, as far south as Chesapeake Bay (2021 fathoms, fide Verrill), 

 off Nova Zembla and the Murman coast. Pteraster caribbseus, from 

 the West Indian area. Pteraster danse, off the east coast of Pata- 

 gonia (or (?) Eio Janeiro). Pteraster sordidus is an Atlantic species 

 mentioned by Perrier by name only, but of which no description is 

 published. 



