124 LUNG-FISHES 
was similar to Dipterus in its skeletal characters. Its elon- 
gate diphycercal tail was continuous with the dorsal and 
anal (?) elements; in this, and in the retention of marginal 
cusp-like teeth, it resembled the Pleuracanthid sharks. 
Living Forms 
The three forms of living lung-fishes may reasonably be 
looked upon as the survivors of the more generalized Palzo- 
zoic forms. Ceratodus, the 
Australian genus, appears to 
have retained most perfectly 
the ancestral conditions; it 
has probably remained almost 
unmodified from the early 
Mesozoic times,* and presents 
close affinities to the Coal 
Measure family, Crenodontide, 
and even to the Devonian 
Dipterids. Its outward ap- 
pearance is shown in Fig. 
127, and its skeleton in Fig. 
128. The latter is seen to 
resemble closely the charac- 
Fig. 128A.— Skull of Ceratodus, : : : 
Seen from the ventral side. (After ters of Fig. 122; its paired 
ATE) fins are archipterygial; the 
c. Occipital rib, d. Dental plates. 
na. Anterior and posterior nares. P. mouth is lacking in marginal 
Palatine. PSph. Paraspenoid. 4. Z . 
Pterygoid. Qu. Quadrate. Vo. Vomer, Cutting plates (cf. V, Fig. 
122 A). The dental plates 
of the palatine and splenial regions (Fig. 128 A) are seen 
to correspond clearly with those of Figs. 125, 125 A. 
Ceratodus had long been known to the colonists of 
*v.p.10. The recent genus, according to Dr. Gill, is to be distinguished 
as Neoceratodus. 
