ON DIPTEEUS AND CTENODUS. 399 



are now in a position to point out other features that separate 

 Dipterus from Ctenodus. In the former the upper dental plates 

 are each adherent to the anterior extremities of the two bones 

 named by Pander respectively the palatal and inner pterygoid. 

 These are elongated flattened bones, and lie parallel to and in 

 close contact with each other, being apparently united by a 

 suture. These conjoined bones form on either side of the oral 

 roof a flattened widish plate, with the posterior extremity some- 

 what expanded. The two plates are united in front along the 

 longitudinal middle line, and diverge widely backwards. 



Now the upper dental plates of Ctenodus are usually found 

 attached to similar plate-like bones, which incline or diverge 

 to the right or left accordingly as they are united to the right 

 or left dental plate. These bony supports (PI. XIV., c, d) are 

 expanded at the posterior extremity, and are in general configu- 

 ration exactly like the plate composed of the palatal and 

 pterygoid bones in Dipterus. Indeed there can be no doubt 

 that they are homologous ; but the fact to which we wish to 

 draw attention is, that while in Ctenodus the component bones 

 are united so as to form but one bone, the suture being entirely 

 obliterated, they are distinctly separated in Dipterus by a well- 

 marked sutural line. This difference would seem to be of some 

 importance ; for it appears to be constant, as it occurs in all 

 our species, of the whole of which, excepting C. corrugatus, 

 these bones have been found. 



The sphenoid bones in the two genera are likewise very 

 different in character. In Dipterus, according to the same high 

 authority already quoted, as well as according to Hugh Miller*, 

 this bone is widely lozenge-formed, and does not extend back- 

 wards very much beyond the extremities of the palato-pterygoid 

 bones to which the dental plates arc attached, and fills up the 

 entire space between them : this bone Pander considers the 

 basi-sphenoid. 



In Ctenodus the sphenoid is a much elongated depressed 

 bone, with a wide lozenge-formed expansion near the anterior 

 extremity. In other words, the posterior angle of the lozenge 



* Sec " Footprints of the Creator," p. 58. 



