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BIPTERIDJE. 
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Order I. SIRENOIDEI. ^ A^/ 
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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- 
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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 . . Dipteridje (p. 235). 
Jugular plates; marginal teeth. Phaneropleurid^ (p. 246). 
No jugular plates ; no marginal teeth. Ctenodontid^ (p. 250). 
Cranial roof-bones few. 
No jugular plates; no marginal teeth. Lepidosirenid.se (p. 264). 
B 
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Family DIPTERID2E. 
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 . . , . 
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Genus DIPTERUS, Sedgwick & Murchison, 
[Trans. Geol. Soc. [2] vol. iii. 1828, p. 143.] ^r^v/ 7 . . 
Syn. Catopterus, L. Agassiz, Poiss. Foss. vol. ii. pt. i. 1833, p. 3. ^ ^ -fy , 
Polyphr actus, , L. Agassiz, Poiss. Foss. V. G. R. 1844, pp. 5, 29. ^ r^'yyj 
Body elongate, not much laterally compressed, covered with 
enamelled cycloid scales; head depressed, snout obtuse. Dental JfJh 
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^ y> */^<3 
anal fins, separated from the caudal. 
The most complete account of the skeletal anatomy of Dipterus 
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