XXII 
IXTRODFCTIOX. 
degenerate survivors. It is equally impossible to justify the 
conception of the groups u Lepidosteoidei ” and “ Amioidei,” most 
of the extinct fishes which are still commonly ascribed to the 
former being proved in the Catalogue to be much more closely 
related to the latter. 
This result is merely the logical outcome of the new departure 
in Ichthyology made by Traquair in 1876, 1 when he published his 
classic memoir on the systematic position of the Palaeoniscidae. 
The characters which Traquairs researches have proved to be 
secondary in the group to which these typically Palaeozoic fishes 
belong, are similarly shown to be of comparatively small taxonomic 
value in the higher groups of Mesozoic age to which the present 
volume specially refers. It therefore seems unnecessary to recapi¬ 
tulate and discuss the various classifications which have been 
proposed; for nearly all of them, even those published during the 
last fifteen years, fail to distinguish between the fundamental and 
the secondary features, which the memoirs cited have made quite 
clear. 
It only remains to add that, in a work of this kind, a more or 
less definite judgment must sometimes be pronounced in cases 
where the known facts scarcely suffice for more than premature 
speculation; and it is always difficult to express a tentative result 
in terms which cannot be exaggerated in quotation. The present 
volume, like its predecessors, is therefore to be accepted merely as 
a convenient basis for further research, full of imperfections which 
each specialist will readily discover for himself. A few partly 
novel conceptions have been introduced, to direct attention to 
certain problems which seem hitherto to have been inadequately 
considered. A careful examination of nearly all the available 
specimens has enabled the writer to attempt a more detailed 
account of the osteology of the principal genera than has previously 
been published. Tor continued generous co-operation, thanks are 
not only due to the many scientific colleagues at home and abroad, 
to whom indebtedness has already been expressed in the previous 
volumes ; they are likewise gratefully tendered to the Government 
1 E. H. Traquair, ‘ The Ganoid Fishes of the British Carboniferous Forma¬ 
tions,’ Part I. (Mon. Palseont. Soc., 1876). See also Memoir on Platysomidm 
already quoted, and “Beport on Fossil Fishes collected by the Geological 
Survey of Scotland in Eskdale and Liddesdale,” Trans. Boy. Soc. Edinb. 
vol. xxx. pp. 15-71, pis. i.-vi. (1881), and “ Notes on Chondrosteus acijpen- 
seroides, Agassiz,” Geol. Mag. [3] vol. iv. pp. 248-257, figs. 1-5 (1887). 
