ON DIPTEEUS AND CTENODUS. 407 



The new Australian fish is described to have two "incisor" 

 teeth in the upper jaw, placed a little in advance of the dental 

 plates. There is no reason for believing that such additional 

 teeth are present in either Dipterus or Ctenodus. Several entire 

 heads of the former have been obtained ; and we possess in the 

 specimen before alluded to of C. elegans a crushed head of that 

 species, and have also two crushed heads of C. obliquus ; and in 

 neither genus has there been found the least trace of any such 

 "incisor" teeth. The four dental plates only are present — two 

 palatal, two mandibular. And, again, these plates are not by 

 any means uncommon at Newsham, where upwards of four 

 hundred specimens have been obtained by Mr. Atthey. Had 

 such "incisors" existed, about two hundred of them might 

 therefore have been expected to occur ; not one has been found. 



This peculiar character alone would seem sufficient to separate 

 generically the so-called Ceratodus Forsteri from .Dipterus and 

 Ctenodus, and shows very clearly the relationship of the former 

 to Lepidosiren, which is provided with two small pointed teeth 

 in front of the upper dental plates,* which latter do not differ 

 much from those of this interesting Australian fish. 



EXPLANATION OF THE PLATES. 

 PLATE XIII. 



Fig. 1. Mandible, natural size, with the dental plate attached, of Ctenodus 

 imbricatus : a, dental plate ; 6, glenoid notch. 



Fig. 2. Outside view, natural size, of the right ramus, with the dental 

 plate attached, of Ctenodus obliquus : a, dental plate ; b, syni - 

 physial margin ; c, glenoid notch ; d, channel or cavity overhung 

 by the dental plate. 



Fig. 3. Scale, much enlarged, of Ctenodus elegans : a, posterior or imbri- 

 cated extremity. 



PLATE XIV. 



The palato-pterygoid bones, natural size, with dental plate attached, of 

 Ctenodus tuberculatus : o, anterior extremity of the bone ; b, den- 

 tal plate ; c, palatal side of the bone ; d, pterygoid side of ditto. 



* "Description of the Lepidosiren annectens," by Ridiard Owen, Esq. Trans. Linn. Soc, 

 Vol. XVIII., p. 341, tab. 27, fig. 2. 



