(4) Acanthodii.
Suborder (1). Ichthyotomi[30].
The members of this suborder range from the Devonian to the Permian and so have long been extinct.
The endoskeletal cartilage has granular calcifications evenly distributed throughout it. The notochord is unconstricted, but the neural and haemal arches are well-developed, and the neural spines are long and slender. There is a continuous dorsal fin with separate basalia and radiale. The tail is diphycercal, and the pectoral fins are typical archipterygia[31]. The pelvic fins of the male are prolonged to form claspers.
The best known of these primitive Elasmobranchs are the Pleuracanthidae.
Suborder (2). Pleuropterygii.
This suborder was formed for the reception of Cladoselache, an Elasmobranch found in the Lower Carboniferous of Ohio[32].
The exoskeleton is in the form of small, thickly-studded dermal denticles. The vertebral centra are unossified, and the tail is strongly heterocercal. There were certainly five, perhaps seven gill slits, and the suspensorium is apparently hyostylic. The paired fins are, according to the view which derives them by concentration from continuous lateral folds, the most primitive known (see p. 129) and claspers are absent.
Suborder (3). Selachii.
Cartilaginous or partially calcified biconcave vertebrae are always well developed; they constrict the notochord intervertebrally. The neural and haemal arches and spines are stout and intercalary cartilages (interdorsalia) are present. The tail is heterocercal, but in some cases (Squatina) approaches the diphycercal condition. In most cases the suspensorium is hyostylic, the jaws being attached to the cranium by means of the hyomandibular, and the palato-pterygo-quadrate bar not being fused to the cranium. There are generally five pairs of branchial arches, and gill rays are borne on the posterior surface of the hyoid arch, and on both the anterior and posterior surfaces of the first four branchial arches. The Notidanidae differ from most Selachians in two respects, first as regards the suspensorium,—Meckel's cartilage articulating directly with the palato-pterygo-quadrate bar, and not being connected with the hyoid arch; and secondly as regards the number of branchial arches,—six pairs occurring in Hexanchus and seven in Heptanchus.