INTRODUCTION. 
During the Mesozoic Period the Actinopterygian fishes of the 
Chondrostean type were gradually replaced by others which advanced 
towards the modern Teleostean type. The latter finally became 
dominant in the seas of the epoch of the Chalk. In the course of 
this change in the character of the fish-fauna, the successive stages 
were represented by numerous minor modifications, which have 
little relation to the direct line of progress but present many 
features of great morphological interest. It is the object of the 
present volume to trace these phases of development in detail, and 
to record in as nearly a natural order as possible the variations of 
each at the time of its dominance. 
Chondrostei. 
Firstly, however, there is the question of the fate of the Chondrostei 
themselves, which are represented even at the present day by a 
few survivors. 
The origin of these fishes, as defined in the present Catalogue, is 
still entirely obscure. Among known fossils they range downwards 
as far as the Crossopterygians, while there is as yet no evidence of 
a link between these two groups. On the other hand, it is clear 
that the Chondrostean is later than the Crossopterygian type; for 
the former is represented in the Devonian solely by the rare genus 
Cheirolepis, while the latter is dominant throughout, and the 
members of the Chondrostei do not flourish vigorously until those 
of the Crossopterygii begin to decline in the Carboniferous and 
Permian. The modifications by which a Crossopterygian could be 
changed into a Chondrostean are also readily comprehensible. In 
the latter the paired fins are always polybasal, with excessively 
shortened lobe; and among Crossopterygii the genera with most 
