being covered with a tuberculated enamel; and, as in the existing Sturgeons, the mouth seems to have been wholly destitute of teeth.  Somewhat allied, also, to the Sturgeons, is a singular group of armoured fishes, which is highly characteristic of the Devonian of Britain and Europe, and less so of that of America.  In these curious forms the head and front extremity of the body were protected by a buckler composed of large enamelled plates, more or less firmly united to one another; whilst the hinder end of the body was naked, or was protected with small scales.  Some forms of this group—­such as Pteraspis and Coccosteus—­date from the Upper Silurian; but they attain their maximum in the Devonian, and none of them are known to pass upwards into the overlying Carboniferous rocks.  Amongst the most characteristic forms of this group may be mentioned Cephalaspis (fig. 103) and Pterichthys (fig. 104).  In the former of these the head-shield is of a crescentic shape, having its hinder angles produced backwards into long “horns,” giving it the shape of a “saddler’s knife.”  No teeth have been discovered; but the body was covered with small ganoid scales, and there was an unsymmetrical tail-fin.  In Pterichthys—­which, like the preceding, was first brought to light by the labours of Hugh Miller—­the whole of the head and the front part of the body were defended by a buckler of firmly-united enamelled plates, whilst the rest of the body was covered with small scales.  The form of the “pectoral fins” was quite unique—­these having the shape of two long, curved spines, somewhat like wings, covered by finely-tuberculated ganoid plates.  All the preceding forms of this group are of small size; but few fishes, living or extinct, could rival the proportions of the great Dinichthys, referred to this family by Newberry.  In this huge fish (fig. 102, a) the head alone is over three feet in length, and the body is supposed to have been twenty-five or thirty feet long.  The head was protected by a massive cuirass of bony plates firmly articulated together, but the hinder end of the body seems to have been simply enveloped in a leathery skin.  The teeth are of the most formidable description, consisting in both jaws of serrated dental plates behind, and in front of enormous conical tusks (fig. 102, a).  Though immensely larger, the teeth of Dinichthys present a curious resemblance to those of the existing Mud-fishes (Lepidosiren).

In another great group of Devonian Ganoids, we meet with fishes more or less closely allied to the living Polypteri (fig. 105) of the Nile and Senegal.  In this group (fig. 106) the pectoral fins consist of a central scaly lobe carrying the fin-rays on both sides, the scales being sometimes rounded and overlapping (fig. 106), or more commonly rhomboidal and placed edge to edge (fig. 105, A).  Numerous forms of these “Fringe-finned” Ganoids occur in the Devonian strata, such as Holoptychius, Glyotoloemus,