ORDERS OF FISHES. 169 



from the Triassic rocks, the true relations of which were 

 then, and for long after, quite obscure. We now know, 

 however, from the happy discovery of the living Ceratodits 

 Forsteri, that these teeth must have belonged to a Dipnoan 



Fig. 535. — A, Dental plate of Ceratodus serratus — Keuper ; b, Dental plate of 

 Ceratodvs alius— Kewper. (After Agassiz.) 



fish, which must have resembled Zejndosiren in many respects, 

 especially in its horny cycloidal scales, and its symmetrical 

 tail and undivided dorsal fin, but which approached the 

 Crossopterygious Ganoids in the fact that the fin-rays of the 

 paired fins were arranged round a central scaly lobe. So 

 far as is yet known, the genus Ceratodus does not occur in 

 any deposit of Palaeozoic age ; but several species are known 

 from the Trias, and a smaller number from the Jurassic 

 rocks. 



On the other hand, the section of the Ctenodipterines — 

 distinguished from the preceding chiefly by the heterocercal 

 form of the tail, the division of the dorsal fin into two, the 

 possession of enamelled scales and cranial plates, and the 

 existence of " gular plates " resembling those of the Cross- 

 opterygious Ganoids — has a much higher antiquity, not only 

 dating from the Devonian, but being, so far as known, wholly 

 Palaeozoic in its range. The type-genus is Dipterus (fig. 

 536, a), of the Old Eed Sandstone, in which the body is 

 covered with cycloidal enamelled scales, the tail is extremely 

 heterocercal, the skeleton is notochordal, and the pectoral 

 fins are acutely lobate. The dental apparatus consists of 

 two triangular, convex, ridged or tuberculated plates attached 

 to the lower jaw (fig. 536, b), and of a pair of similar plates, 

 which are attached to the roof of the mouth in the middle 

 line. So far as is known, the genus Dipterus is exclusively 

 confined to the Old Pted Sandstone period. In the genus 



