The lower Acanthodeans, according to Woodward, "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. In Climatius, one of the most primitive genera (see Fig. 305), there exists, according to Woodward, and as first noticed by Cope, between the pectoral and pelvic (or ventral) 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 this point have not yet been discovered. However, it is evident that dermal calcifications attained a greater development in the Acanthodei than in any of the more typical Elasmobranchs, and we may look for much additional information on the subject when the great fishes to which the undetermined Ichthyodorulites pertained became known." (See Fig. 305.)
The Acanthodei constitute three families. In the Acanthoessidæ there is but one short dorsal fin opposite the anal, and clavicular bones are absent. The gill-openings being provided with "frills" or collar-like margins, perhaps resembled those of the living genus Chlamydoselachus, the frilled shark. The pectoral spine is very strong, and about the eye is a ring of four plates. The body is elongate, tapering, and compressed. Acanthoessus of Agassiz, the name later changed by its author to Acanthodes, is the principal genus, found in the Devonian and Carboniferous.
The species of Acanthoessus are all small fishes rarely more than a foot long, with very small teeth or none, and with the skin well armed with a coat-of-mail. Acanthoessus bronni is the one longest known. In the earliest species known, from the Devonian, the ventral fins are almost as large as the pectorals and nearly midway between pectorals and anal. In the later species the pectoral fins become gradually larger and the ventrals move forward. In the Permian species the pectorals are enormous.
Traquairia pygmæa, from the Permian of Bohemia, is a diminutive sharklet three or four inches long with large scales, slender spines, and apparently no ventral fins.
In the genus Cheiracanthus the dorsal fin is placed before the anal. In Acanthodopsis the teeth are few, large, and triangular, and the fin-spines relatively large.
The Ischnacanthidæ have no clavicles, and two dorsal fins. Ischnacanthus gracilis of the Devonian has a few large conical teeth with small cusps between them.
The Diplacanthidæ, with two dorsal fins, possess bones interpreted as clavicles. The teeth are minute or absent. In Diplacanthus striatus and Diplacanthus longispinus of the Lower Devonian stout spines are attached to the shoulder-girdle between the pectoral spines below.
Fig. 304.—Diplacanthus crassissimus Duff. Devonian. Family Diplacanthidæ. (After Nicholson). (Restoration of jaws and gill-openings; after Traquair.)
In the very small sharks called Climatius the fin-spines are very strong, and a series of several free spines occurs, as above stated, on each side between the pectoral and ventral fins, a supposed trace of a former lateral fold. In Paraxus the first dorsal spine is enormously enlarged in size, the other spines remaining much as in Climatius.