02^ THE GENUS CTElS^ODrS. 109 



G. caudatiis, W. J. Barkas. Prom the Northumberland Coal 

 Measures. The specimen that I pourtray in Pig. IX is the only 

 one that has been discovered as yet. It resembles C. ellipticus 

 to a certain extent, but it differs in being smaller and in having 

 a long projection from one extremity ; this prolongation is not 

 the result of a fracture nor is it a fold of shale, for I have freed 

 the tooth completely from the matrix of shale ; the posterior 

 ridges are rather indistinct, having probably been worn away 

 during life ; but traces are left of four non-tuberculated ridges ; 

 the anterior ridge is broad, much inclined forwards, and projects 

 beyond the outer margin ; the under surface is smooth, and a ridge 

 runs horizontally along its centre from the back of the prolonga- 

 tion ; the tooth is 1 inch long, including the tail, and \ of an 

 inch broad, 



I have now described the characters of the teeth of every 

 species that has been discovered, and with which I am acquainted. 

 In my next paper I shall refer to the microscopical structure of 

 these teeth, and illustrate it by drawings taken with the camera, 

 Portunately all the teeth I have described possess the same 

 structure, there will, therefore, not be any necessity to go over 

 all the species in detail again. My further programme is to 

 pourtray the incisive or vomerine teeth, the mandibular, palatal, 

 and articular bones, and the dental arrangement ; next, the head 

 bones, ribs, operculse, &c., &c. By this means all that is known 

 of Ctenodus will be brought together for the first time into a 

 series of consecutive papers, and comparisons with the similar 

 parts of Geratodus will then be more easily made. 



In order that comparisons may be at once instituted between 

 the teeth of Gtenodus and those of a recent Geratodus, I here- 

 with append Dr. Giinther's description of the external characters of 

 the teeth of the latter fish, which appeared in a paper on Geratodus 

 in the " Philosophical Transactions," Part II, for 1871 : — 



" Each maxillary dental plate is an oblong piece, with a grind- 

 ing surface, a convex inner side, and with the outer side divided 

 into six prominent trenchant ridges or prongs, by five notches, 

 of which the foremost is the deepest, the others becoming 

 shallower posteriorly. The foremost ridge passes to the inner 

 border of the tooth, which is likewise somewhat raised. The 

 grinding surface has a great number of minute depressions or 

 punctuations. The total length of a maxillary tooth is li 

 inch, and its greatest width \ an inch. In form and size the 

 mandibulary teeth are very similar to the maxillary, only the 

 grinding surface is less uneven. These teeth are anchylosed to 

 the bone, and inserted in an oblique direction — the upper teeth 

 nearly meet each other in the median line, but there is rather a 

 wide interspace between the lower." Pig. X. is a maxillary 

 tooth copied from Giinther's monograph, plate XXXIV, fig. 3. 



