PALEOZOIC TIME DEVONIAN. 



589 



head-shield of another large Dipnoan; its length in some specimens is 16 

 inches, indicating a fish of large size. The peculiar spine-bearing plate 

 shown in Fig. 889 is of uncertain relations. 



Ganoids existed of formidable size and dental armature. In one of them, 

 Onychodus sigmoides, the mandible, or jaw (Fig. 890), was 14 inches long, 

 and the head 18 inches. At the extremity of the lower jaw there were very 



890-892. 



Ganoids. — Fig. 890, mandible of Onychodus sigmoides (xj); 890 a, one of the large teeth at the extremity 

 of the mandible (x f). Chlm^eoid Selachian. — Fig. 891, Rhynchodus secans, upper tooth; 891 a, b, 

 extremities of upper and lower mandibles in natural position. Fin-spine of a Selachian. — Fig. 892, 

 Machaeracanthus sulcatus (x |). Figs. 890, 891, Newberry ; 892, Hall. 



few very large teeth; and one of them, nearly half the natural size, is 

 represented in Fig. 890 a. 



To the Chimaeroids are referred the species of Rhyncliodus, having 4 

 large, beak-like teeth, two in each jaw. One of these teeth is shown, natural 

 size, in Fig. 891. The relative positions of the upper and lower jaws at the 

 extremity is shown in Figs. 891 a, h. 



The Selachians or Sharks of Cestraciont type were represented by species 

 of Psammodus. One of the spines of a Shark, probably from the dorsal fin, 

 is represented in Fig. 892 ; the length of this spine is 4 to 6 inches ; but that 

 of another Ohio species is 20 inches. 



The Nevada Devonian. — In the Devonian of Kevada, where a limestone 

 6000 feet thick represents nearly the whole era, the Lower, Middle, and 

 Upper Devonian, out of the 144 species described by Walcott, more than 

 half occur also in the New York Devonian, a number in the Iowa that are 



