518 FISHES CHAP. 
which disappears shortly before the larval metamorphosis. At 
that period the circulation in the cutaneous gills becomes 
sluggish, and very soon these organs completely atrophy. About 
the same time the hyo-branchial cleft closes up, as in Proto- 
pterus. The young Lepidosiren soon begins to breathe air and 
to become more active and lively in its habits... The adult may 
attain a length of four feet. 
The relations of the different genera of Dipneusti to one 
another has been discussed by Dollo in a remarkably suggestive 
paper.” Until the publication of this treatise it was generally 
believed that the modern Dipneusti, Neoceratodus, Protopterus, 
and Lepidosiren, especially the first mentioned, were the most 
é. 
pl. 
Fic. 311.—Larval Lepidosiren thirty days after hatching. ce, Cement organ; c.g, cutaneous 
gills ; p./, pectoral limb ; pv./, pelvic limb. (From Graham Kerr.) 
primitive and the more nearly related to the ancestral stock, 
while the older types, such as Dipterus, were regarded in the 
light of highly specialised offshoots. The continuity of the 
median fins, the apparently diphycercal character of the tail, and 
the wholly cartilaginous condition of the chondrocranium in the 
modern Dipneusti, were contrasted with the divided median fins, 
the heterocercal tail, and the more extensively ossified chondro- 
cranium of the Palaeozoic forms, and the belief seemed inevitable. 
« Dollo has shown, however, that there is good reason for the 
view that the evolution of the group has taken place in exactly 
the opposite direction ; that, in fact, the older Dipneusti are the 
more archaic, and that their modern representatives have been 
derived from them by a sequence of retrogressive changes; or, in 
other words, the latter have much the same relation to the 
former as the degenerate Sturgeons and Paddle-Fishes to their 
Palaeozoic ancestors, the Palaeoniscidae. Taking Dipterus, the 
' For further information about the development of Lepidosiren, see Graham 
Kerr’s valuable paper, op. cit. 
* Dollo, Sur la Phylogénie des Dipneustes, Bruxelles, 1895. 
