INTRODUCTION. 



The present Part, which comprises only the order Chelonia, needs 

 but little in the way of introduction. 



The author must, however, express his indebtedness to Mr. 

 Boulenger's Catalogue of recent Chelonians l , without the aid of 

 which it would have been quite impossible to have attained even 

 such an approximation to a satisfactory arrangement of the fossil 

 forms as is exhibited in the following pages 2 . To bring the 

 classification of the fossil species into accordance with that adopted 

 by Mr. Boulenger for the existing Chelonians, it has been, found 

 necessary to use generic terms in a much more restricted sense 

 than is the case in the previous Part, where, having only extinct 

 types to deal with, there was no obligation to follow any such 

 model. 



It should be observed in regard to the division of the Chelonia 

 into two suborders that the author has, as a provisional measure, 

 followed Messrs. Cope and Boulenger, who regard the Athecata as 

 generalized types widely separated from all the other members of 

 the order. Dr. Baur, however, considers that they are a specialized 

 branch so closely connected with the Chelonidce through the Pro- 

 tostegidce that there is no justification for their subordinal separation. 

 If the skull referred by this writer to Protostega be rightly assigned, 

 there will be evidence of a closer connection between the two groups 

 than has hitherto been supposed. If it could be absolutely proved 

 that Psejohoderma is not really Chelonian, one objection to Dr. Baur's 

 views would be removed. In the event of this view proving to be 



1 Catalogue of the Chelonians, Khynchocephalians, and Crocodiles in the 

 British Museum (Natural History), by Gr. A. Eoulenger (1889). 



2 The author also desires to express his obligations to Mr. Boulenger for 

 much advice during the progress of this Catalogue. 



