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 Pishes, 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 suffices, 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 Loderlein 1 , Koken 2 , Fritsch 3 , and Newberry 4 5 . 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, 
Acanthodes , by Eoemer 0 , 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. Gres, naturf. Freunde Berlin, 1889, pp. 77-94. 
3 A. Fritsch, ‘ Fauna der Gfaskohle,’ vol. ii. pt. iv., vol. iii. pt. i. (1889-90). 
4 J. S’. 'Newberry, ‘ Palteozoic Fishes N. America ’ (1889). 
5 F. Roemer, Zeitschr. deutsch, geol. Gres. vol. ix. (1857), pp. 65-83, pi. iii, 
® C. Liitken, Palasontogr. vol xxii. (1873), p, 41. 
