DEVONIAN AND OLD RED PERIOD. 155 



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 preced- 

 ing, was first brought to light by the labors of Hugh Miller 

 the whole of the head and the front part of the body were de- 

 fended 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 



Fig. 103. Cephalaspia Lyellii. Old Red Sandstone, Scotland. (After Page.) 



this family by Newberry. In this huge fish (fig. 102, a) the 

 head alone is over three feet in length, and the body is sup- 

 posed to have been twenty-five or thirty feet long. The head 

 was protected by a massive cuirass of bony plates firmly articu- 

 lated 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 coni- 

 cal tusks (fig. 102, a). Though immensely larger, the teeth of 

 Dinichthys present a curious resemblance to those of the exist- 

 ing 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- 



