OIS" THE DENTAL BOSES OE CIEIN'ODUS. 128 



The auterior extremity is bluntly pointed and projects beyond 

 the general body of the bone ; from this process the outer 

 bordex' runs in a direction outwards and backwards and joins the 

 posterior margin in a rounded point ; this border is - ry 

 irregular in outline from its alternate concave and convex 

 curves, the bends themselves varying much in their degree. The 

 posterior border presents two gentle concavities, which join a 

 little to the inner side of the centre of the margin. The inner 

 border presents anteriorly a surface for union with the similar 

 portion of the opposing bone ; behind this it forms a broad 

 sweeping conca\dty outwards and backwards, and then a slight 

 convexity just before its junction v/ith the posterior border. 

 The under surface bears upon its outer and anterior portion 

 the dental plate ; which plate projects somewhat beyond the 

 bone, more especially at its posterior extremity. When the two 

 pterygo-palatine bones are in position they are united anteriorly 

 by a long suture, they tlien diverge widely in their progress 

 backwards. In all these points this bone corresponds closely to 

 the ptery go -palatine bone of Ceratodus, with the exception that 

 the latter is hardly so expanded posteriorly. The peculiarity of 

 this bone (and also that of Geratodus) is that it occupies the 

 position of two bones, the palatal and inner pterygoid, and it 

 presents greatly the form that would accrue on the union of 

 these two bones as they are observed in the fossil fish Dipterus. 

 I may remark in jDassing, that Diiiterus possesses many charac- 

 ters in common with Ctenodus and Ceratodus, more particu- 

 larty as regards its teeth ; but in the formation of its upper jaw 

 there are, as I have remarked, two distinct bones which are 

 united by a suture. The form of this bone in Ctenodus and 

 Geratodus, and the fact of the similarity that it has to the 

 united palatine and pterygoid bones of Dipterus, conclusively 

 prove that Dr. Giinther was right in his conjecture that these 

 two bones have been merged into one in the case of Geratodus, 

 and, as Mr. Atthey first pointed out, in the case of Ctenodus. 



As I have not a ptery go -palatine bone with me at present, I 

 have had to trespass again upon the drawings of my friend, Mr. 

 Atthey. The engra\dng appeared in vol. iv of the Transactions 

 of the JN'orthumberland and Durliam Natural History Society. 



[Plates.] 



