216 Notes on Acanthodian Sharks 
suggest a spine-like fin support (Fig. 20). Indeed, in such a form as 
Diplacanthus tenuistriatus “ (Fig. 22) such a condition is actually at- 
tained. And the dermal elements arrange themselves in the series of 
rows characteristic of Acanthodian spines, while the hinder radials be- 
come obsolescent. If, accordingly, such a condition be compared with 
that shown in the second dorsal fin of Huthacanthus macnicoli” (Fig. 
23), we cannot fail to note the close correspondence; the fin spine is 
made up of radial components in which dermal tubercles are concen- 
trated, and ithese even appear behind the fin spine strengthening a series 
of delicate rays which are evidently comparable with the obsolescent rays 
shown in the caudal fin of Fig. 22. A second dorsal spine of an 
Acanthodian is thus the homologue of the concentrated radials in a 
caudal fin. And it is easy to see how such a condition would readily 
become highly specialized in forms in which dermal structures present a 
wide range in their scheme of evolution. In such a broad spine, for 
example, as that shown in Fig. 26, Climatius reticulatus,” the broad 
striation in its dermal crust still testifies to the component radial ele- 
ments,—the basal elements, or at least some of them, forming the region 
b. In Fig. 24, Climatius uncinatus, a more highly modified type, there 
can still be seen traces of the component radials in a spine which has 
become curved. In other spines (Fig. 19) the dermal elements have 
completely obscured the core of radial elements over which primitively 
they have been laid down. 
Admitting, then, that the fin spines of Acanthodians are founded 
upon the concentrated radials of a Cladoselachian fin, the puzzle of the 
paired fins of the Acanthodians becomes greatly simplified. And for a 
further study of the matter, I believe that an important hint is given us 
in the behavior of the dermal elements as contributing to the formation of 
fin spines. For if dermal elements play so important a part in strength- 
ening the outer supports of fins, may they not also have pressed deeply 
into the integument and strengthened the basals? For it is these ele- 
ments which have contributed no little confusion in the studies of the 
Acanthodian paired limbs. Without attempting to reconcile the differ- 
ences which occur in the plan of the shoulder girdle and spines of various 
Acanthodians, I may point out that there is evidence not merely of a pair 
Edinburgh Museum. 
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