as DIPNEUSTI 51D 
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 :—Dipterus, Scawmenacia, 
Phaneropleuron, Uronemus, Ceratodus (Neoceratodus), Protopterus 
and Lepidosiren. Briefly, the more important structural modi- 
fications observable in the transition from the older to the recent 
genera are (@) the gradual union of isolated median fins to 
form a continuous fin”; (6) the substitution of a gephyrocercal 
tail for a heterocercal”; (c) the degeneration of the squamation, 
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; (¢) 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 Crossopterygu with 
acutely lobate fins, or, with greater probability, 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. Reports, 1900, p. 776 et seg. 
2 Compare Figs. 301 and 304. 
3 It is worthy of note that Protopterus 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). 
