5l8 FISHES 



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, Neocercttodus, Protopterus, 

 and Lepidosiren, especially the first mentioned, were the most 



Fig. 311. — Larval Lepidosiren thirty days after hatching, c, Cement organ ; eg, cutaneous 

 gills ; p. I, pectoral limb ; pv,l, pelvic limb. (From Graham Kerr.) 



primitive and the more nearly related to the ancestral stock, 

 while the older types, such as Dip)terus, 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. cii. 



^ Dollo, Sm- la Phyloginie des Dipnexistes, Bruxelles, 1895. 



