Fig. 300.—Cladoselache fyleri (Newberry), restored. Upper Devonian of Ohio. (After Dean.)
Order Acanthodei.—Near the Pleuropterygii, although much more highly developed, we may note the strange group of Acanthodei (ἀκανθώδης, spinous). These armed fishes were once placed among the Crossopterygians, but there seems no doubt that Woodward is right in regarding them as a highly specialized aberrant offshoot of the primitive sharks. In this group the paired fins consist each of a single stout spine, nearly or quite destitute of other rays. A similar spine is placed in front of the dorsal fin and one in front of the anal. According to Dean these spines are each produced by the growing together of all the fin-rays normally belonging to the fin, a view of their morphology not universally accepted.
Fig. 301.—Cladoselache fyleri (Newberry), restored. Ventral view. (After Dean.)
Fig. 302.—Teeth of Cladoselache fyleri (Newberry). (After Dean.)
Fig. 303.—Acanthoessus wardi (Egerton). Carboniferous. Family Acanthoessidæ. (After Woodward.)
The dermal covering is highly specialized, the shagreen denticles being much enlarged and thickened, often set in little squares suggesting a checker-board. The skull is covered with small bony plates and membrane bones form a sort of ring about the eye. The teeth are few, large, and "degenerate in their fibrous structure." Some of the species have certainly no teeth at all. The tail is always heterocercal, or bent upward at tip as in the Cladoselache, not diphycercal, tapering and horizontal as in the Ichthyotomi.