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But the generic character of Zeugopterus, which is most important for the present discussion, is the prolongation of the dorsal and ventral fins on to the lower of the body at the base of the tail, the attachments of these accessory portions being transverse to the axis of the body.
With regard to the specific characters of the species of Zeugopterus nothing is known of peculiarities in mode of life which would give an importance in the struggle for existence to the concrescence of the pelvic fins with the ventral in punctatus, to the absence of this character and the elongation of the first dorsal ray in unimaculatus, or to the absence of both characters in norvegicus.
There is, however, reason to conclude that the organs, especially on the head, are more important and larger in deeper water, and thus the enlargement of the sensory canals in the head of the Witch, which lives in deeper water than other species, may be an adaptive character. Another genus of whose characters I once made a special study is that named Zeugopterus.
If we consider three examples enlarged hind-legs for jumping as in kangaroo or frog, absence of colour from the lower sides of Flat-fishes, and, thirdly, the finlets on the lower side of Zeugopterus we see that they represent three different kinds of characters, all related to habits or external conditions.
We may say that the third kind are correlated with some other character that has a relation to function or external conditions, as the extension of the fins on the under side of Zeugopterus is correlated with the enlargement of the fins, whose function is to cause the adhesion of the fish to a vertical surface.
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