Comprises the oldest vertebrate remains, from Devonian and Carboniferous formations. _Pterichthys_: (Figs. 135 and 136), tail tapering, covered with small ganoid scales, without caudal fin; the cephalic shield was probably moveably joined to the cuirass of the trunk, and both were composed of several pieces; the abdominal shield consisted of one single median plate, and two pairs of lateral plates, a third small pair being sometimes observed detached in front of the anterior pair; pectoral exceedingly long, consisting of two pieces movably connected with each other; tail scaly, and short; a small dorsal fin placed on the tail; a pair of small ventrals; jaws small, with confluent denticles. Several species have been distinguished in remains found in the strata of Caithness and other localities in Scotland. _Coccosteus_ (Fig. 137, p. 354): all the bony plates are firmly united, no pectoral spines; tail naked and long; a dorsal and anal fin supported by interneural and interhæmal spines. Dentition unknown. _Dinichthys_: a gigantic fish from the Devonian of North America (estimated at from 15 to 18 feet in length), with the dermal covering very similar to that of Coccosteus, but with a simple arched dorsal shield. As in this latter genus the caudal extremity does not possess external or internal bony parts, and the ventral plastron of both genera corresponds in every particular; the dentition is so singularly like that of Lepidosiren, that Newberry (Geolog. Survey of Ohio, vol. ii. part 2) considers this genus to be in genetic relation to the Dipnoi. The following genera have been united in a separate family, _Cephalaspidæ_; viz. _Cephal__aspis_: head covered by a continuous shield with tubercular surface, produced into a horn at each posterior corner; a median dorsal backward prolongation bears a spine; heterocercal. _Auchenaspis_ and _Didymaspis_: allied to the preceding, but with the cephalic shield divided into a larger anterior and smaller posterior piece. _Pteraspis_: with the cephalic shield finely striated or grooved, composed of seven pieces. _Scaphaspis_ and _Cyathaspis_: with the surface of the head-shield similarly sculptured as in _Pteraspis_, but simple in the former, and composed of four pieces in the latter. _Astrolepis_: attained to the gigantic size of between twenty and thirty feet; its mouth was furnished with two rows of teeth, of which the outer ones were small, the inner much larger. [See Ray Lankester, A. Monograph of the Fishes of the Old Red Sandstone of Britain. Part I. Cephalaspidæ. Lond. 1868 and 1870. 4to.] [Illustration: Fig. 135.--Dorsal surface of Pterichthys, after Pander. _d_, Dorsal fin; _c_, pectoral member; 2–10, head-bucklers; 11–13, dorsal bucklers.] [Illustration: Fig. 136.--Ventral aspect of Pterichthys, after Pander. 15, mandible (?); 16–21, ventral bucklers.]