﻿INTRODUCTION. XV 



loped among the Acanthodii is, indeed, a singular and interesting 

 feature. So far as the observations recorded in the following pages 

 have extended, such elements only occur in the head when tho 

 dentition is still preserved. In the lower jaw there is a bone pro- 

 bably corresponding to the splenial ; and in the upper jaw there is 

 an ensheathing element in connection with each half of the pterygo- 

 quadrate arcade. In the pectoral arch, again, membrane- calcifica- 

 tions have only been noticed when there are great dermal spines to 

 be supported. Two elements, occupying the position of clavicle and 

 infraclavicle, are especially conspicuous in the formidably armed 

 Diplacanthus (see p. 23) ; Avhile in the comparatively feeble types 

 of Acanthodidae and Ischnacanthidaa, such calcifications are either 

 insignificant or absent. Under any circumstances the development 

 of membrane-elements in the Acanthodii cannot be regarded as 

 more than a family character ; and it is a striking illustration of 

 the now generally received principle, that features which become of 

 wide taxonomic importance in the higher groups are sporadic and 

 of small significance on their first appearance in the lower groups. 



Lastly, it may be remarked that, notwithstanding the extreme 

 specialization of the paired limbs, the lower Acanthodians are the 

 only vertebrates in which there are any structures in the adult, 

 apart from the two pairs of fins, which may be plausibly interpreted 

 as remnants of once-continuous lateral folds 1 . As observed by 

 Prof. Cope 2 , the earliest known members of the order (e. g. Cli- 

 matius) exhibit between the pectoral and pelvic fins a close and 

 regular series of paired spines, in every respect identical with those 

 supporting the appendages .that presumably correspond to the two 

 pairs of fins in the higher genera. They may even have supported 

 fin-membranes, though specimens sufficiently well preserved to 

 determine the point have not yet been discovered. However, it is 

 evident that dermal calcifications attained a greater development in 

 the Acanthodii than in any of the more typical Elasmobranchs : and 

 much additional information on the subject may be expected when 

 the great fishes to which some of the undetermined Ichthyodorulites 

 pertained become known. 



1 We do not overlook the theory of the rudimentary third pair of limbs in 

 Callorhynchus (T. J. Parker, ' Nature,' vol. xxxiy. 1886, p. 635). 



2 E. D. Cope, Amer. Nat. vol. xxiv. (1890), p. 407. 



