392 THE VOYAGE OF H.M.S. CHALLENGEK. 



Thus, then, thirteen species have been found in the abyssal zone, two of which are also 

 littoral, while three are continental. The two former both belong to the genus Rliizociinus, 

 of which no exclusively abyssal species are known ; though it has been met with at sixteen 

 out of the thirty-four Stations in the abyssal zone. It is well represented in the Lower 

 Tertiaries, and perhaps ranges back to the Cretaceous period, when its larger ally 

 Bourgueticrinus was so abundant. 



Pentacrinus has been found at nine Stations where the depth exceeded 500 fathoms ; 

 and two of its four abyssal species are also continental. But on the other hand, 

 Bathycrinus, which occurs in the Atlantic at nine abyssal Stations between lat. 65° N. 

 and 46° S., has never been found at a less depth than 1050 fathoms ; while it embraces 

 four out of the eight species which are peculiar to the abyssal zone. 



No fossil Bathycrinus is known, hoR^ever, and the genus has no special affinities 

 except with Rhizocrinus, of which it may almost be said to be the "benthal" ^ representative. 

 Of the four remaining abyssal species, one is the sole representative of the remarkable 

 genus Hyocrinus, and has only been met with at 1600 fathoms and still greater depths. 

 Like the Comatulid genus Thauinatocrinus, which occurs at 1800 fathoms in the Southern 

 Ocean, it has certain strong points of resemblance to the Palseocrinoids. 



Pentacrinus ranges back to the Trias and Rhizocrinus to the Eocene or Upper 

 Cretaceous. But they are both abundant at depths of less than 100 fathoms, Pentacrinus 

 occurring in the Pacific and in the East Indian Archipelago, as weU as in the Atlantic and 

 among the Caribbean Islands ; while Rhizocrinus, though limited to the eastern 

 hemisphere, ranges through over 100° of latitude. 



In spite, therefore, of the existence of a few characteristic abyssal types, it is some- 

 what of an exaggeration to speak of the Stalked Crinoids as a group " on the verge 

 of extinction," of which a few survivors may occasionally be discovered in the deeper 

 parts of the great ocean basins. 



^ Dr. Gwyn Jeffreys has suggested that this word be employed to denote depths exceeding 1000 fathoms. See 

 hisadcbess to the Biological Section at the Plymouth Meeting of the British Association, 1877, p. 79. 



