﻿RHIZODONTID^E. 341 



Family RHIZODONTTDjE. 



Body fusiform, robust, elongated, and somewhat depressed, with 

 cycloidal scales, more or less deeply overlapping, exhibiting a 

 rounded boss or short rib on the middle of the inner side, and 

 sometimes covered externally with a thin layer or detached rugae 

 of ganoine. Head and opercular apparatus with well-developed 

 membrane-bones ; parietals large and separate, frontals separate, 

 and orbits far forwards ; interoperculum absent ; jugular plates 

 comprising one large pair, flanked on either side by a lateral series, 

 and with a small azygous element in front. Dentary bone of man- 

 dible thin and vertical, with well-developed infradentaries in the 

 same plane; an inner series of a few large, narrow, shuttle-shaped 

 bones, each supporting a " laniary " tooth ; a pair of similar teeth 

 on the roof of the mouth, but the marginal upper dentition feeble. 

 Teeth conical, with a pulp-cavity of which the walls are vertically 

 folded towards the base. Pectoral and pelvic fins obtusely lobate ; 

 two remote dorsal fins, the first nearly opposite or directly opposed 

 to the pelvic pair : anal fin single, caudal fin diphycercal or 

 heterocercal. 



Synopsis of Genera. 



I. Infraclavicle with long upwardly directed 



process. 

 Teeth smooth, with a pair of sharp edges. Rhizodus (p. 342). 

 Teeth rounded in section Strepsodus (p. 348). 



II. Infraclavicle without an ascending pro- 



cess; dorsal fins directly opposed to 

 pelvic and anal fins. 



Teeth rounded in section, smooth ; ring- 

 vertebrae ; tail heterocercal, and cau- 

 dal fin rhomboidal. Rhizodopsis (p. 354). 



No ring- vertebrae ; tail almost diphycercal, 



and caudal fin rhomboidal Gyroptychius (p. 358). 



Teeth rounded in section ; ring-vertebrae ; 



tail almost diphycercal and truncated. Tristichopterus (p. 360). 



Teeth compressed, with a pair of sharp 

 edges ; ring-vertebrae ; tail hetero- 

 cercal and truncated Eusthenopteron (p. 361). 



