No. 391.] B/POLARITY OF MARINE FAUNAS. 589 
along the western coast of America to 22° N. L., but there it 
has been dredged at greater depths (1200-1400 m.). Although 
no species of this group has yet been found in the northern 
Pacific, it is very probable, nevertheless, that such species do 
exist. In that case we would here have again an instance of a 
connection, along the western coast of America, of an apparently 
bipolar group of marine animals. Examples of this kind have 
been pointed out, by the present writer, among the decapods. 
Of the 6 ophiuroid genera common to both the polar seas, 4 
(Ophioglypha, Ophiactis, Amphiura, Ophiacantha)are also repre- 
sented in the littoral of the tropics. The 2 remaining genera 
(Ophiocten, Gorgonocephalus) are chiefly abyssal genera, and, 
as far as the writer could determine, Ludwig does not give any 
information — at least, Ophiocten is also found in tropical 
latitudes. 
Thus in these three groups we have again the same result : 
that bipolarity, if present at all, is extremely rare, and that the 
most prominent feature of the respective faunas of the polar 
seas consists in their dissimilarity. 
Yet Ludwig calls attention to a certain general likeness of 
both faunas, expressed by the mutual prevalence of certain 
genera and the mutual lack of others as compared with the 
tropic faunas. This is not to be regarded at all as a remark- 
able fact, and has no connection with the question under dis- 
cussion ; indeed, it would be very strange if other conditions 
prevailed. When, out of a number of genera present in the 
tropics, a certain number disappears as we approach either 
pole, while a certain number does not, this shows only that 
the latter are not affected by the change of conditions, —chiefly 
climatic, —while the former are, and of course by the disappear- 
ance of a number of types the percentage of the remaining must 
increase, if the deficiency is not made up by other genera 
making their appearance in the colder regions. This is again 
an instance where statistics give a wrong idea of the true con- 
ditions; the increase of the percentage of certain genera in the 
polar seas is not due to an actual increase of species and a more 
vigorous development, but only to the lack of species of other 
genera. 
