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



Colour in alcohol, a bleached yellowish white. The large niadreporiform 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 Pterasterid.e, 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 Pterasteridse 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 ; Bentliaster, 

 two ; Calyptraster, one ; and Pythonaster, one. 



Two new genera have been recently added to the family by Perrier, viz., Myxaster 1 

 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. — For 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 Pterasteridse, 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 paxillas, 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 spiracula by Sars. A large aperture occurs in the 



1 Cowptes remdus, 1885 (November), t. ci. p. 886. 



2 Ann. Sci. Nat. (Zool.), 1885, 6e Sene, t. xix., art. No. 8, p. 69. 



