Dean on Acanthodei.—In his latest treatise on these fishes, "The Devonian Lamprey," Dr. Dean unites the Pleuropterygii and Acanthodei in a single order under the former name, regarding Acanthoessus as an ally and perhaps descendant of the primitive Cladoselache. Dr. Dean observes:
"In the foregoing classification it will be noted that the Acanthodia are regarded as included under the first order of sharks, Pleuropterygii. To this arrangement Smith Woodward has already objected that the spines of Acanthodians cannot be regarded as the homologues of the radial elements of the Cladoselachian fin (which by a process of concrescence have become fused in its interior margin), since he believes the structure to be entirely dermal in origin. His criticism, however, does not seem to me to be well grounded, for, although all will admit that Acanthodian spines have become incrusted, and deeply incrusted, with a purely dermal calcification, it does not follow that the interior of the spine has not had primitively a non-dermal core. That the concrescence of the radial supporting elements of the fin took place pari passu with the development of a strengthening dermal support of the fin margin was the view expressly formulated in my previous paper on this subject. It is of interest in this connection to recall that the earliest types of Acanthodian spines were the widest, and those which, in spite of their incasing dermal calcification, suggest most clearly the parallel elements representing the component radial supports. There should also be recalled the many features in which the Acanthodians have been shown to resemble Cladoselache."
Fig. 305.—Climatius scutiger Egerton, restored. Family Diplacanthidæ. (After Powrie, per Zittel.)
From these primitive extinct types of shark we may proceed to those forms which have representatives among living fishes. From Cladoselache a fairly direct series extends through the Notidani and Cestraciontes, culminating in the Lamnoid and Galeoid sharks.
Still another series, destitute of anal fin, probably arising near the Acanthodei, reaches its highest development in the side branch of the Batoidei or rays. The Holocephali and Dipneusti must also find their origin in some of these primitive types, certainly not in any form of more highly specialized sharks.
Fig. 306.—Pleuracanthus decheni Goldfuss. Family Pleuracanthidæ. (After Roemer, per Zittel.)
Woodward prefers to place the Tectospondyli next to the Ichthyotomi, leaving the specialized sharks to be treated later. There is, however, no linear system which can interpret natural affinities, and we follow custom in placing the dogfishes and rays at the end of the shark series.