NORTH PACIFIC OPHIURANS IN NATIONAL MUSEUM CLARK. 19 



very different from each other, and the same is true of the two species 

 of Ophiomitra, the two of OpJiioihrix, and the three of Ophiacantha. 



Sagami Bay has long been known as a rich collecting ground, and 

 the Albatross found numerous ophiurans there. At the entrance to 

 the Gulf of Tokio stations 5091-5095, in water 58 to 302 fathoms deep, 

 with a bottom temperature of 57.8-43.9, twenty-four species were 

 collected, of which three are species of Ophiacantha, three belong to 

 OpJiiopholis, four to Ophiomusium, and six to Ophiura. The most 

 notable cases of closely allied species in this group are those of Ophiura 

 micracantha, sarsii, and flagellata, and of Ophiopholis mirabilis and 

 aculeata. 



But the best way in which to decide whether closely allied species 

 occur together is to take certain well-defined species whose nearest 

 allies are also North Pacific forms and see whether the two species 

 often or ever occur at the same place. It is not always possible to 

 say what other species is the nearest ally of any given species ; such a 

 point is necessarily largely a matter of personal opinion. But for 

 any fair discussion of associational distribution it is sufficient if the 

 two species are closely enough allied so that one might naturally have 

 arisen from the other. 



The species of the genus Ophiura form a large proportion of the 

 ophiuran fauna of the North Pacific, and of these species sarsii is by 

 far the commonest and most widely distributed. A very closely 

 related species, liiikeni, is also common in the eastern Pacific and 

 there is little doubt that sarsii is the nearest relative of lutkeni. 

 There is no possible question, however, of their specific distinctness, 

 for the difference between them is obvious and remarkably constant. 

 Yet these two species were taken at the same station seven times 

 (stations 2858, 2862, 2882, 3047, 3053, 3059, and Sitka) and twice 

 they were taken at adjoining stations where conditions were essen- 

 tially identical (stations 2867 and 2868, and 2883 and 2884). It is 

 clear, therefore, that they are not geographically or bathymetrically 

 isolated from each other. 



Another equally interesting illustration of the same facts in the same 

 genus is shown by the species leptoctenia and quadrispina; the latter 

 certainly seems to find in the former its nearest ally, yet the difference 

 between the two is sharp enough to prevent any difficulty of identi- 

 fication. The two species were taken together at stations 3331, 3332, 

 3337, 4770, 4781, 4791, and 5026. Another species (micracantha) is 

 also very closely allied to sarsii and leptoctenia; it was taken at station 

 5091, while sarsii occurred close by under identical conditions at 

 station 5092, and leptoctenia not very far away at station 5083. 



The two species of the new genus Opliiopenia were taken twice at 

 the same station (at Captains Harbor, Unalaska, and at station 2854), 

 and there can be no possible doubt that they are much more nearly 

 allied to each other than to any other known ophiurans. 



