MONOGRAPH 



OF THE 



SIRENOID AND CR0SS0PTERY6IAN GANOIDS. 



This treatise attempts to collect into one view the most important facts which have 

 been ascertained respecting the organisation and classification of at least the more typical 

 Ganoid fishes. To compare in their principal features similar forms of all geological 

 periods is the author's object, and not the detailed investigation of a particular fauna. It 

 will therefore be necessary to leave to the compilers of more special memoirs all struc- 

 tural details of little physiological or morphological value, while the present arrangement 

 will be zoological and not stratigraphical. 



The practice of the Palaeontographical Society, no less than the local restriction of an 

 author, requires that the British species should receive the chief attention. This limita- 

 tion is not so damaging to the usefulness of a memoir on Ganoid fishes as might at first 

 sight appear. British rocks have yielded a very considerable proportion of the genera 

 hitherto defined, and nearly all the chief modifications of Ganoid structure can be fairly 

 illustrated by British examples. It will be necessary to introduce genera wholly foreign 

 at times ; in particular, the recent Ganoids, none of which are strictly British, will require 

 full consideration. 



The known Ganoidei are already very numerous, and it would be unwise to count 

 upon the time necessary for their complete revision. I have therefore definitely under- 

 taken only two sections of the order, but it is my present intention to review, sooner or 

 later, the remaining suborders. 



Useful general principles relating to the distribution and derivation of animal types 

 will, it is hoped, come to light in the progress of these studies, and this expectation is 

 the chief motive for undertaking so lengthy and troublesome a work. The difficult 

 inquiry, " What is a Ganoid ?" may also receive elucidation. It would be proper to 

 discuss this fundamental question at once, if the order were throughout its history so 

 sharply defined as to render serious mistakes improbable. Unfortunately, this is not the 



A 



