DIPNEUSTI 519 



3 



most ancient of all the known Dipneusti, as a starting-point, it 

 is possible to select a series of genera which illustrate the evolu- 

 tion of the group both in structure and in palaeontological 

 sequence.'' The series is as follows : — JDipteriis, Scaumenada, 

 Phaneropleuron, Uronemus, Ceratoclus {Neoceratochis), Protopterus 

 and Lepidosiren. Briefly, the more important structural modi- 

 fications observable in the transition from the older to the recent 

 genera are (a) the gradual union of isolated median fins to 

 form a continuous fin " ; (b) the substitution of a gephyrocercal 

 tail for a heterocercal " ; (c) the degeneration of the squaniation, 

 the thick ganoid scales of the earlier types being replaced by 

 thin, non-ganoid scales ; (d) a reduction in the number of 

 cranial dermal bones and the loss of their original ganoid invest- 

 ment ; (e) the suppression of the jugular plates ; and (/) a 

 reduction in the size of the opercular bones. In the last two 

 genera of the series, in which specialisation in some respects and 

 degeneration in others have reached their maximum, the body 

 no longer retains the fusiform and more typically Fish-like shape 

 of the older genera, but, in accordance with Eel-like habits and 

 mode of progression, has become more or less Eel-like in form. 

 The paired fins are almost vestigial, while the scales, so deeply 

 insunken in the skin as to be externally invisible, suggest that 

 the modern Dipneusti are approximating to a final scaleless 

 as well as to an ultimately limbless condition. As to the origin 

 of the Dipneusti as a group, it seems reasonable to look for 

 their ancestors in the early Devonian Crossopterygii with 

 acutely lobate fins, or, with greater probabiHty, to some still more 

 primitive Crossopterygian with simple, non - rhizodont teeth, 

 capable by fusion of giving rise to massive tritoral plates, and 

 involving as a consequence the substitution of an autostylic for 

 an originally hyostylic skull, and the suppression of the secondary 

 upper jaw. In fact, when our knowledge of the development of 

 the surviving Dipneusti and Crossopterygii is more complete, it 

 is not improbable that the inclusion of the two series of Fishes in 

 subordinate divisions of the Teleostomi will prove to be amply 

 justified. The relations of the Dipneusti to the Amphibia are 



1 For critical remarks, see Traquair, Brit. Ass. Reporta, 1900, p. 776 et seq. 



2 Compare Figs. 301 and 304. 



^ It is worthy of note that ProlopUrus dolloi approaches Lepidosiren in the more 

 Eel-like shape of its body, and in the large number of pairs of ribs (54) which it 

 possesses (Boulenger, op. cit. p. 37). 



