X 
INTRODUCTION. 
geology has as yet revealed nothing definite concerning the fresh- 
water life of the Cretaceous and Jurassic periods. In the case of 
the fishes of the suborder Ostariophysi this is especially unfortunate; 
because it is clear that in early Tertiary times they were already 
well differentiated, and it is probable that their peculiarly modified 
auditory apparatus was originally an adaptation to the circum- 
stances of life in a confined area. Even the highly specialised 
Siluroids were completely developed in the early part of the Eocene 
period ( Bucklandium , p. 830) ; and one Eocene skull is identical 
with that of the existing genus Arias (p. 333). The Characinida) 
have a cranium not much more advanced than that of Amia ; and 
as these seem to be at the baso of the series of Ostariophysi, the 
modification of the auditory apparatus must have begun in a very 
low type of physostomous fish. 
Apodes. 
The eels are unique among the so-called toleostean fishes in 
possessing more than five basal bones in the pectoral fin — a feature 
characteristic of all the lower groups of Actinopterygii. Their 
skull is also of a comparatively primitive type. The elongate shapo 
of theii body, the loss of the upper part of their pectoral arch, and 
the complete loss of their pelvic and caudal fins, aro marks of 
degenerate specialisation. Even before the end of the Cretaceous 
peiiod they displayed all these characteristics oxcept the lack of 
the caudal fin ; for Urenchelys (p. 337), from the Chalk of Mount 
Lebanon, is a typical generalised eel only differing from the 
geneialiscd existing genera in the possession of a small separate 
caudal fin. It therefore seems probable that, although the. Apodes 
me 1 eleostei in the sense in which these are defined by Muller 
and Gunther, they aro not degenerate members of any group of 
tin, sc modern fishes, but aro directly derived from some of the 
Mesozoic fishes which would be termed “ Ganoidei ” by the same 
authors. At least they cannot be regarded as degenerate members 
any group of Cretaceous “ Teleostei ” hitherto discovered. 
Pekcesoces. 
Spinous fin-rays, except as mere basal fulcra, are unknown 
e oic the Cretaceous period; and tho earliest type of fish in which 
y form a conspicuous feature of the dorsal fin is Crossoynat A*** 
(p. 3-18), from the Ncocomian of Switzerland and Germany.’ In it 8 
