SHALLOW-WATER STARFISHES 369 



Hippasteria hyadesi Per. 



326 meters. 

 Ceramaster patagonicus (Sladen). 



55 to 245 fathoms. 

 C. austro-granularis (Per., 1891, as Pentagonaster). 



340 meters. 

 Leptychaster kerguelenensis Smith. 



Kerguelen and Marion Islands ; 10 to 100 fathoms. 

 L. antarcticus Sladen. 



210 fathoms. 

 Psilaster Heuriaisi (Per., i8gi, as Goniopecten). 



198 to 283 meters. 

 Cheiraster gerlachei Lud. 



450 to 560 meters. 

 Pontaster planeta Sladen. 



245 fathoms. 

 Ctenodiscus australis Liitk. 



55 to 600 fathoms. 

 C. procurator Sladen. 



40 to 1325 fathoms. 



All the species (thirty-four) included in this list are generically 

 allied to northern or arctic species. In many cases the resemblance is 

 very close specifically. This is so especially in the cases of some 

 species of Henricia, Solaster, Pteraster, Diplopteraster, Porania, 

 Hippasteria, Ceramaster, and Ctenodiscus. 



These facts clearly indicate that extensive migrations of the ances- 

 tors of these species have occurred, in the colder deep waters, in one 

 or both directions, at no very remote geologic periods. 



It appears, with our present knowledge of oceanic currents and 

 temperatures, that the greater part of the migration would have been 

 from the Arctic Ocean southward to the Antarctic. But no barriers 

 exist, at the depths that these species inhabit, to prevent a northward 

 migration, for abyssal species are not included in the list. The shal- 

 low-water and littoral species, on the other hand, show no evidence 

 of such comparatively recent migrations, for much the larger part 

 are peculiar to this region and the adjacent Antarctic seas and islands, 

 with a few additions from the Indo-Pacific region. 



Scarcely any of the strictly shallow-water species seem to have 

 been able to cross the tropical regions, on either side of America, 

 unless in remote geologic time. 



Those that can descend to the depths where very cold water exists, 

 even in tropical seas, have been able to migrate from polar seas across 

 the tropics, probably in both directions, no doubt with extreme slow- 

 ness, and yet with but little change in appearance or structure. Such 



25 



