X 
INTRODUCTION. 
membrane-bones, which have a definite symmetrical arrangement, 
and consist, at least in their basal layer, of tissue with distinct 
lacunae, these being often arranged in haversian systems. All 
palaeontological 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 
