470 THE VOYAGE OF H.M.S. CHALLENGER. 
Three pairs of secondary mouth-spines, free and inde- 
pendent : : : ‘ : : : Calyptraster. 
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 paxille short, not protruding through, but supporting, 
the membrane Hymenaster. 
6, Nidamental cavity aborted. Supradorsal membrane rudimentary. 
No muscular fibres. No spiracula. Spinelets of paxille 
fascicular, protruding a great portion of their length naked 
through the membrane : : : : : . E . Benthaster. 
B. No supradorsal membrane present. No actino-lateral spines. No seg- 
mental apertures. Form stellate, rays greatly produced, attenuate 
and cylindrical. Abactinal spines short, in sheathed fascicules, not 
forming pedicellate paxille . 2 : : : : 3 . PYTHONASTERINA. 
a, A single genus , . : ; : 4 , ; : Pythonaster. 
Subfamily PrerasTeRIn&, Sladen, 1888. 
Genus Pteraster, Miller and Troschel. 
Pteraster, Miiller and Troschel, System der Asteriden, 1842, p. 128. 
With the exception of two Atlantic species, Pteraster caribbzus 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. 
a. Geographical distribution :— 
ATLANTIC: Five species between the parallels of 80° N. and 60° S. 
*Pteraster mulitaris, off the Norwegian, British, and North 
American coasts, extending up Smith’s Sound as far as Dobbin Bay, 
and south to Cape Cod, off Spitzbergen and Nova Zembla, and in the 
Kara Sea. Pteraster pulvillus, 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 caribbeus, from 
the West Indian area. Pteraster dane, off the east coast of Pata- 
gonia (or (?) Rio Janeiro). Pteraster sordidus is an Atlantic species 
mentioned by Perrier by name only, but of which no description is 
published. 
