574 PISCES—FISHES. 
recall Crossopterygii; in their cartilaginous skeleton 
and persistent notochord they are 
primitive ; in their lung, heart, in- 
ferior vena cava, multicellular skin- 
glands, and eggs they approach 
Amphibians. 
The Dipnoi are physiologically 
transitional between Fishes and 
Amphibians, having, for instance, 
acquired lungs while retaining gills, 
but it does not follow that they 
are morphologically transitional. 
They are intermediate, but that is 
not to say that they are ¢Ze connect- 
ing links. 
Ceratodus.— The genus Cera- 
todus is abundantly represented by 
fossils in the Mesozoic beds of 
Europe, America, Asia, and Aus- 
tralia, but the living animal is now 
limited to the basins of the Burnett 
and Mary rivers of Queensland 
(see Fig. 6). Like that other old- 
fashioned animal the duckmole, 
Ceratodus frequents the still, deep 
places of the river’s bed, the 
so-called ‘water-holes.” At the 
bottom of these it lies sluggishly, 
occasionally rising to the surface 
to gulp in air. Its diet was for- 
merly supposed to be exclusively 
vegetarian, but Semon holds that 
it crops the luxuriant vegetation of 
the river-banks only for the sake 
of the associated animal life— 
larvee and eggs of insects, worms, 
molluscs, amphibians, and _ fishes. 
Though Ceratodus is quite unable 
to live out of water, its air-breath- 
ing powers enable it to exist in 
water which is laden with sand or rotten vegetable matter. 
ie: 
\) 
" 
Fic. 307. —Lepidosiren (after Graham Kerr), showing (7¢,/.) pectoral fin and the 
tufted pelvic fin (Pv.f.) of the mature male. 
