468 THE VOYAGE OF H.M.S. CHALLENGER. 
Colour in alcohol, a bleached yellowish white. The large madreporiform body is con- 
spicuous by its slightly orange shade of light brown. 
Locality.—Station 304. Off the Peninsula of Tres Montes, west coast of Patagonia. 
December 31,1875. Lat. 46° 53’ 15” S., long. 75° 12’ 0” W. Depth 45 fathoms. Green 
sand. Surface temperature 57°:2 Fahr. 
Family PTERASTERIDA, Perrier, 1875. 
Prior to the Challenger Expedition this family was represented by a very limited 
number of forms, only nine species being on record. Eight of these belonged to the 
genera Pteraster and Retaster, and the ninth was the type and solitary representative of 
Hymenaster, a genus established by Sir Wyville Thomson for a remarkable Asterid dis- 
covered during the cruise of H.M.S. ‘‘ Porcupine.” 
Thirty-five species of Pterasteridze were obtained by the Challenger, only two of which 
were previously known. Of the thirty-three new species, three belong to Pteraster, four to 
Retaster, and the remarkable number of twenty to Hymenaster, a genus which is now 
found to possess a world-wide distribution in deep waters. The remaining six species 
are representatives of four new genera, viz. :—Marsipaster, two species; Benthaster, 
two; Calyptraster, one; and Pythonaster, one. 
Two new genera have been recently added to the family by Perrier, viz., Myaaster’ 
and Cryptaster ;* but the brief notices which are given of these interesting forms are too 
short to enable me to class them satisfactorily in the subjoined synopsis. 
Note on Terminology.—F¥or the sake of brevity, and to avoid verbose repetition, several 
terms are employed in the following descriptions which have not previously been used 
in their present special signification. The introduction of these terms is necessitated 
by structural peculiarities in the forms comprised in the family Pterasteridze, several of 
which have hitherto been unobserved, whilst others have been ignored or passed over by 
previous systematists. The application of the terms will, in most cases, be self-evident. 
The following is a brief definition. 
The supradorsal membrane is the veil-like covering or external independent tissue 
whereby the dorsal nidamental cavity is formed. The’ membrane is supported above the 
true abactinal surface of the animal by the paxille, which consist of a long columnar 
pedicle surmounted by a “crown” of fine, more or less elongate spinelets. In the majority 
of forms belonging to this family, fine muscular fibrous bands extend between the tips 
of the spinelets and constitute a more or less regular fibrous network; and the general 
tissue of the supradorsal membrane which fills in the interspaces or meshes is usually per- 
forated by small contractile pores, styled spzracula by Sars. A large aperture occurs in the 
1 Comptes rendus, 1885 (November), t. ci. p. 886. 
* Ann. Sci. Nat. (Zool.), 1885, 6e Série, t. xix., art. No. 8, p. 69. 
