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 Palaeo- 

 zoic forms. Ceratodns, 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, Ctenodontidce, 

 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- 

 ters of Fig. 122 ; its paired 

 fins are archipterygial ; the 

 mouth is lacking in marginal 

 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. 



Fig. 128 A. — Skull of Ceratodns. 

 Seen from the ventral side. (After 



ZiTTEL.) 



c. Occipital rib. d. Dental plates. 

 na. Anterior and posterior nares. P. 

 Palatine. PSph. Paraspenoid. Ft. 

 Pterygoid. Qu. Quadrate. Vo. Vomer. 



