Fig. 307.—Pleuracanthus decheni, restored. (After Brongniart.) The anterior anal very hypothetical.
Fig. 308.—Head-bones and teeth of Pleuracanthus decheni Goldfuss. (After Davis, per Dean.)
Fig. 309.—Teeth of Didymodus bohemicus Quenstadt. Carboniferous. Family Pleuracanthidæ. (After Zittel)
Order Ichthyotomi.—In the order Ichthyotomi (ἰχθύς, fish; τομός, cutting; named by Cope from the supposed segmentation of the cranium; called by Parker and Haswell Pleuracanthea) the very large pectoral fins are developed each as an archipterygium. Each fin consists of a long segmented axis fringed on one or both sides with fin-rays. The notochord is very simple, scarcely or never constricted, the calcifications of its sheath "arrested at the most primitive or rhachitomous stage, except in the tail." This is the best defined of the orders of sharks, and should perhaps rank rather as a subclass, as the Holocephali. Two families of Ichthyotomi are recognized by Woodward, the Pleuracanthidæ and the Cladodontidæ. In the Pleuracanthidæ the dorsal fin is long and low, continuous from head to tail, and the pectoral rays are in two rows. There is a long barbed spine with two rows of serrations at the nape. The body is slender, not depressed, and probably covered with smooth skin. The teeth have two or more blunt cusps, sometimes with a smaller one between and a blunt button behind. The interneural cartilages are more numerous than the neural spines. The genera are imperfectly known, the skeleton of Pleuracanthus decheni only being well preserved. This is the type of the genus called Xenacanthus which, according to Woodward, is identical with Pleuracanthus, a genus otherwise known from spines only. The denticles on the spine are straight or hooked backward, in Pleuracanthus (lævissimus), the spine being flattened. In Orthacanthus (cylindricus), the spine is cylindrical in section. The species called Dittodus and Didymodus are known from the teeth only. These resemble the teeth of Chlamydoselachus. It is not known that Dittodus possesses the nuchal spine, although detached spines like those of Pleuracanthus lie about in remains called Didymodus in the Permian rocks of Texas. In Dicranodus texensis the palato-quadrate articulates with the postorbital process of the cranium, as in the Hexanchidæ, and the hyomandibular is slender.
Fig. 310.—Shoulder-girdle and pectoral fins of Cladodus neilsoni Traquair.
A genus, Chondrenchelys, from the sub-Carboniferous of Scotland, is supposed to belong to the Pleuracanthidæ, from the resemblance of the skeleton. It has no nuchal spine, and no trace of paired fins is preserved.