344 BULLETIN 16, UNITED STATES NATIONAL MUSEUM. 



KEY TO THE KNOWN GENERA OP PTERASTERID.B. 



a'. Armature of adambulacral plates forming transverse combs, the spinelets united by a web. 

 6'. Supradorsal membrane with muscular fibrous bands; actinolateral spines not completely merged 

 in actinal floor, but the webbed tips forming a free independent ventro-lateral fringe, 

 c'. Muscular bands of supradorsal membrane not conspicuous; usually not regularly reticulated 

 and circumscribing the spiracula, but meandering or crisscrossing, usually very inconspicuous; 

 sometimes spicules are present in supradorsal membrane . Pteraster a M iillerand Troschel, p. 344. 

 c'. Muscle bands conspicuous, regularly reticulated; no supradorsal spicules. 

 d'. Adambulacral plates of two kinds, less prominent alternating with more prominent, and 

 correlated with them two sorts of furrow combs, one longer and set farther into furrow, the 

 other set back slightly; actinolateral membrane broad and fleshy; tube feet in four 



rows Diplopteraster Verrill, p. 370. 



(P. Adambulacral plates not of two kinds; tube feet in two rows; actinolateral membrane 



narrow RetttsterVeTriei. 



V. No muscular fibrous bands in supradorsal membrane; actinolateral spines merged in actinal floor; 

 no lateral fringe, 

 c'. Paxillse spinelets (15 to 30), long and hair-like, protruding freely through the membrane; adam- 

 bulacral armature partly horizontal in disposition; one pair of suboral spines united by web 



to marginal mouth spines Marsipaster Sladen. 



e^. Paxillic spinelets (five or six) short, robust, not protruding; adambulacral armature perpen- 

 dicularin disposition; three pairs of suboral spines, free and independent. . Calyptrasttr S\a,den. 

 a'. Armature of adambulacral plates not forming transverse webbed combs; spinelets free. 

 6'. Nidamental cavity spacious; supradorsal membrane well developed; spinelets of paxillae support- 

 ing the membrane, not protruding through; muscle fibres of supradorsal membrane well devel- 

 oped; spiracula present, 

 c'. Mouth plates large with suboral spines; actinolateral spines long, projecting beyond border 



of ray Hymenasler Thomson, p. 373. 



<?. Mouth plates small, without suboral spines; actinolateral spines short not reaching to interra- 

 dial line; supradorsal membrane regularly reticulated, each mesh containing a spiracle. 



Cryptaster Perrier. 



6^. Nidamental cavity small, the spines projecting through the thin supradorsal membrane which 



lacks or has only very rudimentary muscle fibres Benthaster Sladen. 



Genus PTERASTER Miiller and Troschel. 



Pteraster Miiller and Troschel, System der Asteriden, 1842, p. 128. Type, Asterms militaris 



O. F. MuUer. 

 Hexaster Perrier, Comptes rendus, vol. 112, No. 21, May 1, 1891, p. 1227; M^m. soc. zool. 



France, vol. 4, 1891, p. 267. Type, Hexaster obscurus Perrier. 

 Temnaster Verrill (subgenus), Proc. U. S. Nat. Mus., vol. 17, 1894, p. 275; Amer. Jour. Sci. 



vol. 49, 1905, p. 202 (as genus). Type, Pteraster (Temnaster) hexactis YeTn\\=Pteraster 



ohsaxrus (Perrier). 

 Plerasleridcs Verrill, Amer. Naturalist, vol. 43, Sept., 1909, p. 547, footnote. Type, Pteraster 



aporiLS Ludwig. 



Diagnosis. — Pterasteridae having muscle-bands in the supradorsal membrane, 

 but these as a rule not regularly reticulated; adambulacral armature in the form of 

 transverse webbed combs; actinolateral spines forming a free independent lateral 

 fringe, not merged in actinal floor; supradorsal membrane frequently with small 

 spicules (but not invariably). 



" Lophopteraster Verrill (Amer. Journ. Sci., vol. 49, 1895, p. 202) is stated to differ from Pteraster "in 

 having a very prominent, solid crest or keel-like prominence on the center of each jaw; it forms the 

 inner angle of the jaw, separating the two groups of oral spines in the middle." Otherwise it is like 

 Pteraster. Type, Pteraster abyssorum Verrill. This may be a good genus, but it seems rather slightly 

 differentiated from Pteraster, the shape of the mouth plates constituting the only difference. 



