INTRODUCTION. 
As soon as fishes with a completely osseous endoskeleton began to 
predominate at the dawn of the Cretaceous period, specialisations 
of an entirely new kind were rapidly acquired. Until this time 
the skull of the Actinopterygii had always been remarkably 
uniform in type. The otic region of the cranium often remained 
incompletely ossified, and was never prominent or projecting beyond 
the roof-bones; the supra-occipital bone was always small and 
covered with the superficial plates ; the maxilla invariably formed 
the greater part of the upper jaw ; the cheek-plates were large and 
usually thick; while none of the head or opercular bones were 
provided with spines or ridges. The pelvic fins always retained 
their primitive remote situation, and the fin-rays never became 
spines. During the Cretaceous period the majority of the bony 
fishes began to exhibit modifications in all these characters, and 
the changes occurred so rapidly that, by the dawn of the Eocene 
period, the diversity observable in the dominant fish-fauna was 
much greater than it had ever been before. At this remote epoch, 
indeed, nearly all the great groups of bony fishes, as represented 
in the existing world, were already differentiated, and their 
subsequent modifications have been quite of a minor character. 
It is the object of the present volume to enumerate and partially 
discuss the known Cretaceous and Tertiary bony fishes which 
illustrate these phenomena. The result, however, is much less 
satisfactory than might have been expected from the study of 
animals which lived under conditions most favourable for their 
preservation as fossils. The circumstance that a very large pro¬ 
portion of the Tertiary fishes are known only from detached 
otoliths, suffices to indicate the extreme imperfection of the 
geological record in their case. 
