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 the 
dentition is still preserved. In the lower jaw there is a bone pro¬ 
bably corresponding to the splenial; and in the upper j aw there is 
an ensheathing element in connection with each half oi the p' erygo- 
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 
Dijplacanthus (see p. 23); while in the comparatively feeble types 
of Acanthodidae and Ischnacantbidae,. 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. CU- 
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. xxxiv. 1886, p. 685). 
2 E. D. Cope, Amer. Nat. vol. xxiv. (1890), p. 407. 
