ON. DIPTERUS AND CTENODUS. 405 



So long as Ctenodus and Dipterus were represented merely by 

 the dental plates, they were placed with the Placoids ; and no 

 one would have been justified in placing them elsewhere. There- 

 fore, until some further information is obtained respecting Cera- 

 todus, it would seem best to allow it to remain as at present 

 located, along with the Selachians. The form of its dental plates, 

 too, is sufficiently characteristic to justify its separation generi- 

 cally from this interesting Australian animal, as well as from 

 Dijrterus and Ctenodus. Certainly in the dental organs all three 

 approximate to the Selachians ; but the Ganoid characters so 

 predominate that we apprehend no naturalist would hesitate to 

 place them in that order, though they may be considared to a 

 certain extent " synthetic" forms, as suggested by M. Agassiz.* 



In the present state of knowledge respecting Ceratodus, it is, 

 then, evidently hazardous to place Mr. Krefft's fish in that genus; 

 but its affinity with Dipterus and Ctenodus, more especially with 

 the latter, is clear enough. All three are covered with large cy- 

 cloidal scales ; the fins are arranged much in the same manner ; 

 the skeletons are nearly in the like state of partial ossification ; 

 the dental plates are much alike, there being four ridged plates — 

 two palatal, two mandibular. And when the so-called Ceratodus 

 Forsteri has been fully examined, there can be little doubt that 

 this affinity will become only the more evident. 



Nevertheless the relationship is perhaps closest with Ctenodus. 

 Like most of the members of that genus, the Australian fish is 

 large, measuring from three to six feet in length. And it is only 

 necessary to look to the mandibles (PI. XIII., figs. 1, 2,) of the 

 two forms to be convinced how close this relationship is. In 

 Ctenodus the ramus (fig. 2) is a stoutish bone, flattened vertically, 

 with the upper margin turned over towards the external surface, 

 to give support to the large dental plate; it is therefore chan- 

 nelled on the outer surface, and somewhat convex on the inner. 

 The posterior extremity projects backwards beyond the dental 

 plate a little more than half the length of the latter, and is for 

 the greater part occupied by the glenoid surface, which extends 

 from the upper margin, and is a deep, wide, circular notch, 



* "Nature," No. 61. Vol. III., p. ICG (1870;. 



