NORTH PACIFIC OPHIURANS IN NATIONAL MUSEUM CLARK. 115 



Genus OPHIOPHOLIS. 



The vast amount of material of this genus which has accumulated 

 in the National Museum from the North Pacific makes it possible to 

 offer here a revision of the species. The task has been a very difficult 

 one on account of the extraordinary diversity which individuals show 

 in all those features upon which specific distinctions must be based. 

 Few animals show as great individual diversity in color as do the 

 brittle-stars of this genus; it is scarcely an exaggeration to say that 

 in all the thousands of specimens I have examined I have never seen 

 two that were exactly alike, and there are few shades of color which 

 can not be matched in a living group of the best-known species. 

 The same diversity is seen in the covering of the disk and the sepa- 

 ration of species solely by the number and arrangement of the bare 

 plates is perfectly futile. In the length of the arm spines, the develop- 

 ment of disk spines and the shape of the arm plates, and particularly 

 in the number, form, and arrangement of the supplementary upper 

 arm plates, the same remarkable diversity is found. The examina- 

 tion of more than five thousand specimens of the genus has satisfied 

 me that in Opliiopliolis aculeata we have the primary species of the 

 group, a species with circumpolar distribution and extending far 

 down along the coasts of Europe, Asia, and America, in both the 

 Atlantic and Pacific oceans. For some obscure reason, there has 

 been no formation of new species or varieties in the Atlantic area 

 at least, I can not find any important differences between European 

 and American specimens, nor between those from the northern and 

 southern parts of the American range. In Bering Sea and the north- 

 ern Pacific, however, Ophiopholis seems to have met with very favor- 

 able conditions and has become diversified to such a degree that we 

 can properly recognize at least two very distinct species (mirabilis 

 and brachyactis) , two others (longispina and laJceri) which are fairly 

 well differentiated, and two well-marked varieties which, however, 

 grade into the parent form completely in Bering Sea. These two 

 varieties have already received names from Lyman, who, in the 

 absence of extensive material, regarded them as valid species, distin- 

 guished by the number and arrangement of the disk scales. One of 

 them (kennerlyi, including also caryi Lyman) is the characteristic 

 form of the Pacific coast of the United States, while the other (japo- 

 nica) is apparently the most common form on the coast of Japan. 

 My first inclination was to consider these two forms as subspecies of 

 aculeata, each characteristic of a geographical area, but I soon found 

 that this is not the case, for some of the finest examples of japonica 

 are from Alaska, typical aculeata occurs on the coast of Japan, and 

 some of the best specimens of kennerlyi are from the Aleutian Islands. 

 The typical form was taken by the Fisheries steamer Albatross at 



