The Cladodontidæ differ in having the "pectoral fin developed in the form of a uniserial archipterygium intermediate between the truly biserial one of Pleuracanthus and the pectoral fin of modern sharks." The numerous species are known mainly from detached teeth, especially abundant in America, the earliest being in the Lower Carboniferous. One species, Cladodus nelsoni (Fig. 310), described by Traquair, from the sub-Carboniferous of Scotland shows fairly the structure of the pectoral fin.
Fig. 311.—Teeth of Cladodus striatus Agassiz. (After Davis.) Carboniferous.
In Cladodus mirabilis the teeth are very robust, the crown consisting of a median principal cone and two or three large lateral cones on each side. The cones are fairly striate. In Lambdodus from Illinois there are no lateral cones. Other genera are Dicentrodus, Phœbodus, Carcharopsis, and Hybocladodus.
[CHAPTER XXX]
THE TRUE SHARKS
Order Notidani.—We may recognize as a distinct order, a primitive group of recent sharks, a group of forms finding its natural place somewhere between the Cladoselachidæ and Heterodontidæ, both of which groups long preceded it in geological time.
The name Notidani (Notidanus, νωτιδάνος, dry back, an old name of one of the genera) may be retained for this group, which corresponds to the Diplospondyli of Hasse, the Opistharthri of Gill, and the Protoselachii of Parker and Haswell. The Notidani are characterized by the primitive structure of the spinal column, which is without calcareous matter, the centra being imperfectly developed. There are six or seven branchial arches, and in the typical forms (not in Chlamydoselachus) the palato-quadrate or upper jaw articulates with the postorbital region of the skull. The teeth are of primitive character, of different forms in the same jaw, each with many cusps. The fins are without spines, the pectoral fin having the three basal cartilages (mesopterygium with propterygium and metapterygium) as usual among sharks.