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



in his later work 1 the independence of the genus is recognised, and two new species 

 dredged during the "Talisman" Expedition have been referred by him to Hymenaster 

 and " named," but their descriptions have not yet been published. 



The dredgings of the Challenger Expedition have now shown that Hymenaster 

 possesses a world-wide distribution in deep waters, and that the genus exhibits a 

 remarkable amount of morphological plasticity, no less than twenty-four species being 

 now known. 



The bathymetrical range of the genus is also remarkable, as, with the exception of the 

 type form {Hymenaster pellucidus), which ranges from 70 to 1539 fathoms (fide 

 Danielssen and Koren), all the species are confined to the Abyssal zone. One, Hymenaster 

 infernalis, extends to 2900 fathoms, the greatest depth at which starfishes have hitherto 

 been found ; and four other species occur in depths greater than 2000 fathoms. 



The character of the adambulacral armature, which does not form transverse combs of 

 webbed spines, and the structure of the actinal skeleton, composed of greatly developed 

 actino-lateral spines, overlaid with a membranous tissue, forming the whole actinal 

 floor of the test, constitute differences which, in my opinion, emphatically separate 

 Hymenaster generically from Pteraster ; and its divergence from the other members of 

 the family Pterasteridaa is equally well marked. 



The general facies of the type appears to be one of great antiquity. This, however, 

 is not the place to discuss, as I should desire, the archaic relationships of existing 

 Asterids ; and I would therefore now only briefly direct attention to the remarkable 

 resemblance and, in many respects, apparent similarity of general character, which exist 

 between Hymenaster and the recently described Loriolaster of Stiirtz 2 from the Lower 

 Devonian slates of Bundenbach. 



Clwrology of the Genus Hymenaster. 



« 



a. Geographical distribution : — 



Atlantic : Seven species between the parallels of 81° N. and 40° S. 



Hymenaster pellucidus, from the Faeroe Channel, off the coast 

 of Norway, and the islands of Jan Mayen and Spitsbergen. * Hymen- 

 aster membranaceus, south-west of Cape Finisterre. Hymenaster 

 rex and Hymenaster giboryi, dredged by the " Talisman " Expedition. 

 ^Hymenaster anomalus, north of the Island of Tristan da Cunha. 

 Hymenaster modestus, off the coast of the United States of North 

 America. * Hymenaster pergamentaceus, off the coast of South 

 America, east of Buenos Ayres. 



1 Ann. Sci. Nat. (Zool), 1885, 6e S6rie, vol. xix. Art. No. 8, p. 69. 

 5 Palxontographica, 1886, Bd. xxxii. p. 94. Taf. xii., xiii. 



