Another genus of whose characters I once made a special study is that
named _Zeugopterus._ The name was originally given by Gottsche to the
largest species _Z. punctatus,_ from the fact that the pelvic fins are
united to the ventral, but this character does not occur in other species
now included in the genus. There are three species, occurring only in
European waters, which form this genus and agree in the following
characters. The outline of the body is more nearly rectangular than in
other Flat-fishes from the obtuseness of the snout and caudal end, and the
somewhat uniform breadth of the body. The surface is rough from the
presence of long slender spines on the scales. There is a large
perforation in the septum between the gill cavities, but this occurs also
in _Arnoglossus megastoma,_ which is placed in another genus. 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. These fishes
have the peculiar habit of adhering to the vertical surfaces of sides of
aquaria, even the smooth surfaces of slate or glass. In nature they are
taken occasionally on gravelly or sandy ground, but probably live also
among rocks and adhere to them in the same way as to vertical surfaces in
captivity. Many years ago (_Journ. Mar. Biol. Assn._, vol. iii 1893-95) I
made a careful investigation of the means by which these fishes were able
to adhere to a smooth surface, at least in the case of the largest and
commonest species _Z. punctatus._ It was observed that so long as the fish
was clinging to a vertical surface the posterior parts of the fins were in
rhythmical motion, undulations passing along them in succession from
before backwards, the edge of the body to which they were attached moving
with them. The effect of these movements was to pump out water backwards
from the space between the body and the surface it was clinging to, and to