﻿INTRODUCTION. XI 



endoskeleton, and are correlated instead with the merely dermal 

 developments in the fin-fold itself. Though not absolutely diagnostic, 

 on account of intermediate conditions, the three principal stages in 

 the development of the paired fins correspond closely to three 

 ordinal groups ; while the modifications of the median fins are of 

 less value, sometimes not even diagnostic of divisions which other 

 characters lead to be regarded as suborders. 



Summarizing the present state of knowledge, the subclasses and 

 orders of fishes of which the endoskeleton has been discovered may 

 thus be arranged as in the table on page xii. Another sub- 

 class, that of Ostracodermi, also demands consideration in connection 

 with Palaeozoic Fishes, whether or not jaws and a paired appendi- 

 cular skeleton eventually prove to be absent. All these divisions 

 are defined in the Catalogue itself, and it thus sufiices, by way of 

 introduction, merely to justify some of the features in the arrange- 

 ment adopted, and to particularly emphasize a few of the more 

 important results. 



ELASMOBRANCHII. 



In the Introduction to the first part of the present Catalogue, 

 published nearly two years ago, the chief known features in the 

 palaeontology of the typical Elasmobranch fishes were summarized 

 and discussed ; and subsequent contributions to the subject have 

 been made by Doderlein 1 , Koken 2 , Fritsch 3 , and Newberry 4 . The 

 researches detailed in the following pages make still further addi- 

 tions to existing knowledge of the subclass ; and it now seems 

 possible to recognize a feature of considerable interest that has 

 hitherto escaped adequate notice. This relates to the early speciali- 

 zation of the Elasmobranchii, and the extinction of all but the more 

 generalized types before the end of the Palaeozoic Epoch. 



Such, at least, appears to be the most philosophical interpretation 

 of the characters presented by the remarkable Palaeozoic order of 

 Acanthodii. Since the first detailed description of the typical genus, 

 AcantJwdes, by Roemer 5 , it has been generally admitted that this 

 order of fishes is closely connected with the Elasmobranchii by 

 several important characters, and some authors (e. g. Liitken 6 and 



1 L. Doderlein, Zool. Anzeiger, vol. xii. (1889), pp. 123-127. 



2 E. Koken, Sitzungsb. G-es. naturf. Freunde Berlin, 1889, pp. 77-94. 



3 A. Fritsch, ' Fauna der Graskohle,' vol. ii. pt. iv., vol. iii. pt. i. (1889-90). 



4 J. S. Newberry, ' Palaeozoic Fishes N. America ' (1889). 



5 F. Eoeraer, Zeitsclir. deutscb. geol. Ges. vol. ix. (1857), pp. 65-83, pi. iii. 



6 C. Liitken, Palaeontogr. vol xxii. (1873), p. 41. 



