﻿X INTRODUCTION. 



membrane-bones, which, have a definite symmetrical arrangement, 

 and consist, at least in their basal layer, of tissue with distinct 

 lacuna?, these being often arranged in haversian systems. All 

 pahvontological evidence combines to indicate that both among the 

 hyostylic and the autostylic fishes these two types of exoskeleton 

 have characterized divergent or parallel phyla, exhibiting no con- 

 nection since their origin ; and, if the evolution of the paired fins 

 be regarded as a criterion, three of these four types (i. e., all except 

 the bony hyostylic group) attained their maximum specialization 

 before the end of the Palaeozoic Epoch. 



The evolution of the fins, indeed, and especially of the paired 

 fins, is shown by Cope to be the most satisfactory and philosophical 

 clue to the arrangement of all the minor groups of fishes. Just as 

 the various modifications of the pentadactyl limb in the Ungulate 

 Mammals — the vertebrates which eventually become most com- 

 pletely adapted for progression on land — afford the principal means 

 of determining the natural subdivision of that order ; so among the 

 greater groups of fishes — the vertebrates that become specially 

 adapted for progression in water — the successive modifications of 

 the primitive fin-folds form the most obvious clue to the phases 

 through which the various types have passed in the course of their 

 specialization. 



If, in accordance with the present teaching of embryological re- 

 search, the paired limbs have developed from lateral folds, the 

 primitive condition of these appendages still remains undiscovered, 

 and their evolution can only be traced from a comparatively ad- 

 vanced stage. All the most generalized early Palaeozoic fishes 

 hitherto met with exhibit two pairs of limbs, of the paddle-like 

 form termed " archipterygium " by Gegenbaur : and subsequent 

 specialization has resulted in the gradual atrophy of these limbs, 

 usually with a concomitant development of the fringing dermal 

 rays (actinotrichia). Of the median azygous fin-fold almost the 

 earliest stages are known, and in this case again specialization re- 

 sults, first in the subdivision and partial loss of the originally con- 

 tinuous fold, then in the development of the dermal rays and the 

 gradual atrophy of the endoskeletal supports, and finally in the 

 intimate correlation of these two series of elements. In the most 

 primitive types, there is at least a double series of endoskeletal rods 

 supporting the continuous fin, directly apposed to the neural and 

 haemal spines of the axial skeleton ; in later types the appendicular 

 elements gradually lose all connection with the segments of the 



