DEVONIAN AND OLD RED PERIOD. 153 



of this group are of small size ; but few fishes, living or extinct, 

 could rival the proportions of the great Dinichthys, referred to 



Fig. -iot,.Pterichthys cornutus. Old Red Sandstone, Scotland. (After Agassiz.) 



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- 

 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 HoloptycJiius, Glyptolczmus, Osfcolcpis, Phaneropleuron , &c. 

 To this group is also to be ascribed the huge Onychodus (fig. 

 102, d and e), with its large, rounded, overlapping scales, an 

 inch in diameter, and its powerful pointed teeth. It is to be 

 remembered, however, that some of these "Fringe-finned" 

 Ganoids are probably referable to the small but singular group 

 of the " Mud-fishes" (Dipnoi), represented at the present day 

 by the singular Lepidosircn of South America and Africa, and 

 the Ceratodus of the rivers of Queensland. 



Leaving the Ganoid fishes, it still remains to be noticed that 

 the Devonian deposits have yielded the remains of a number 

 of fishes more or less closely allied to the existing Sharks, 



