﻿MPTERIDrE. 



235 



Order I. SIRENOIDEI. 



Head with well-developed dermal or membrane bones ; principal 

 dentition consisting of triturating plates on the pterygoid and 

 splenial elements. Dermal armour of trunk, when present, con- 

 sisting of imbricating scales ; no plates. Notochord persistent. 

 Paired fins archipterygial ; pelvic arch consisting of a single 

 bilaterally-symmetrical cartilage. 



Synopsis of Families. 



A. Cranial roof-bones numerous. 



Jugular plates ; no marginal teeth . . Dipteeid^e (p. 235). 



Jugular plates ; marginal teeth Phaneropleurid^e (p. 246). 



No jugular plates ; no marginal teeth. Ctenodontidje (p. 250). 



B. Cranial roof-bones few. 



No jugular plates; no marginal teeth. Lepidosirenid.*: (p. 264). 



Family DIPTERID.E. 



Cranial roof-bones numerous ; no distinctly differentiated maxilla 

 or premaxilla, and no marginal series of teeth above or below ; 

 jugular plates present. Caudal fin heterocercal. Scales cycloid. 



The only sufficiently defined genus referable to this family is 

 Dipterus. 



Genus DIPTERUS, Sedgwick & Murchison. 

 [Trans. Geol. Soc. [2] vol. iii. 1828, p. 143.] 



Syn. Catopterus, L. Agassiz, Poiss. Foss. vol. ii. pt. i. 1833, p. 3. 



Polyphractus, L. Agassiz, Poiss. Foss. V. G. R. 1844, pp. 5, 29. 



Body elongate, not much laterally compressed, covered with 

 enamelled cycloid scales; head depressed, snout obtuse. Dental 

 plates, above and below, triangular in shape, with outwardly 

 radiating ridges, tuberculated or strongly crenulated. Paired fins 

 acutely lobate ; two remote dorsal fins opposed to the pelvic and 

 anal fins, separated from the caudal. 



The most complete account of the skeletal anatomy of Dipterus 



