ON DIPTEKUS AND CTENODUS. 401 



are in the other united so that no suture is perceptible. The 

 mandibles (PL XIII., fig. 1) are also very much alike in both 

 genera, and so is their relationship to the dental plates. 



The cranial bones of Ctenodus, so far as we are able to 

 compare them, also closely resemble those of Dipterus, in 

 which the whole of them appear to have been determined. 

 Unfortunately, only those of the posterior part of the skull 

 are known in Ctenodus. A fragmentary specimen of the 

 occipital region of C. tubercidatus in our possession exhibits 

 the occipitals, with a portion of the parietals and the lateral 

 bones, all lying in their natural positions and united into one 

 mass, the component parts being distinguished by well-marked 

 sutures ; and the whole so closely resemble the same bones 

 in Dipterus that they might be taken to belong to a gigantic 

 specimen of that genus. This fragmentary skull is eight inches 

 and a half across the occiput. 



The posterior portions of three other skulls have likewise 

 been found : these belong to C. obliquus ; they are nearly 

 perfect, and one of them displays the bones up to and including 

 the parietals. The surface is in excellent order, and is more 

 coarsely pitted or punctate than in the former species, and 

 exhibits throughout a semigloss ; the sutures are quite distinct. 

 The bones vary in form only specifically from those of 

 C. tubercidatus : the occipital is the most characteristic, the 

 frontal margin of which is deeply concave, while in the latter 

 it projects and has a wedge-shaped process in the centre. 

 This beautiful specimen measures seven inches across. The 

 two other examples agree in all their characters with the 

 above ; but some of the lateral bones of one side are wanting. 

 As in Dipterus, the component bones in Ctenodus are com- 

 paratively small, vary little in size, are many-sided, and fit 

 together like a mosaic pavement or like the pieces of a Chinese 

 puzzle. And that the anterior bones of the skull are of the 

 same character, we have ample evidence in numerous separate 

 bones that have occurred at Newsham, which though differing 

 in form from those composing the fragmentary skulls, un- 

 doubtedly belong to Ctenodus. 



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