122 LUNG-FISHES 
perfect evidence is discovered as to their kinship and the 
lines of their descent. 
In the history of fishes, Dipnoans are known to have been 
early a dominant group. In some regards, one of their 
ancient forms bore many resemblances to the Pleura- 
canthid shark, which, although known at present only in 
a later period, may well have been its contemporary. But 
the range in the forms of Dipnoans occurring in the early 
Palaeozoic indicates the remote antiquity of their origin. 
They had even then evolved exoskeletal characters which 
are scarcely less specialized than those of existing forms. 
' 
\ 
Worst 
7 
ations 
ieee! 
SCC 
Fig. 126. —A restoration of the Devonian lung-fish, Phaneropleuron. X }. 
Dipterus, of the Old Red Sandstone (Fig. 123), had a 
complete body armouring of cycloidal scales, a head roofing 
of dermal plates (Fig. 124), and well-calcified jaw rims — 
(Figs. 124, 125, 125A). Its fin rays were dermal in — 
structure, its paired fins were archipterygial, its tail and 
its dorsal fins separate and lobate. Its mucous canals had 
become elaborately adapted to the body scales (lateral line, 
Fig. 123) and head plates, piercing the latter with minute 
pores, as in Figs. 65,66. Anterior and posterior nares are — 3 . 
indicated under the rim of the upper jaw (Fig. 125, 1-2). 
Marginal teeth have disappeared; a pair of elaborate dental 
plates on the mouth roof (palatine) are apposed by a simi- 
lar pair in the hinder part of the mandible (splenial). 
The Carboniferous Ctenodus was a nearly allied form. 
Another Devonian lung-fish, Phaneropleuron (Fig. 126), 
