144 



ORDERS OF FISHES. 



the Carboniferous rocks, however, occur the superficially 

 similar forms which constitute the genus Hhizodus (fig. 508), 

 in which the teeth agree with those of Holoptychius in 

 being of two sizes, but differ in being trenchant on both 

 sides. The genus, again, differ from Holoptyclims, and ap- 

 j)roaches 3Icgaliclithys, in having an obtusely-lobate pectoral 

 fin, as is also the case in the allied lihkodopsis. lihizodus 

 must have attained a large size, and must have been highly 

 predaceous in its habits. 



"-^(iiiy^^^j^ 



Fig. 508. — Jaw of lihizodus Hibbcrti. Carboniferous. 



We may also provisionally place near Holoptychius the 

 genus Onychodus, from the Devonian of North America. In 

 this genus are Ganoid fishes of large size having the cranium 

 covered with bony plates, the surface of which is enamelled 

 and tuberculated. The jaws carry 

 numerous conical recurved teeth ; and 

 the scales (fig. 509) resemble those of 

 Holoptychius in being cycloidal and 

 overlapping, the under surface con- 

 centrically striated, and the exposed 

 portion of the upper surface adorned 

 with tubercular wrinkles. 



We may further place here the so- 

 Fig. 509 -Upper surface of called " Deudrodout " Ganoids (Den- 



■A scale of Onyclwdus sigmoides, _ ^ 



of tiie natural size, from the dvodus, OHcodus, &c.), which are espe- 



Devonian of North America. .,, t ,• • ^ ii ,^ 0,^1 



(After Newberry.) cially distuiguished by the tact that 



the teeth have a singularly complicated 

 and labyrinthine microscopic structure, somewhat resembling 

 tlie pattern of the teeth in the Amj^hibian order of the 

 Lcibyrinthodontia. The Dendrodonts are Devonian in their 

 rancre. 



