ON THE DENTAL BONES OF CTENODUS. 121 



The mandible or dentary bone is comparatively frequently 

 discovered in the shales of the True Coal Measures of Northum- 

 berland, but no jaw has been brought to light in any other for- 

 mation so far as my knowledge extends, and in all cases the 

 tooth is found still attached to it. In those cases where teeth 

 are discovered unconnected with the mandible (a frequent occur- 

 rence) or pterygo-palatine bone, they show evident traces that 

 they were once anchylosed to the bone, in fact, we saw when 

 examining the teeth microscopically that the osseous structure 

 of the bone gradually merged into the true dental tissue. Mr. 

 Atthey, in a paper that appeared in the Trans. Northd. and Dur- 

 ham Nat. Hist. Socy. vol. iv, states that he has obtained the dentary 

 bones of Ctenodus tuherculatus, C. cristatus, C. obliqims, C. elegans, 

 G. imlricatus, but I myself have only had the opportunity of 

 examining the mandibles of the first and third species named 

 above ; this want of observation is, however, of no moment, as 

 Mr. Atthey distinctly states that the variation in form is very 

 slight, being mostly one of degree in size. Mr. T. P. Barkas, 

 P.G.S., records in different periodicals his discoveries of mandi- 

 bles, and figures one in his book " Coal Measure Palaeontology," 

 but Mr. Atthey, in conjunction with the late Mr. Hancock, were 

 the principal authors on the different parts of the endo-skeleton 

 of Ctenodus, and as their remarks are generally exact, I shall 

 employ their words, for my descriptions could only be similar in 

 regard to the facts. "In Ctenodus {pbliqxms) the ramus is a. 

 stoutish bone, flattened vertically, with the upper margin turned 

 over towards the external surface to give support to the large dental 

 plate ; it is therefore channelled on the outer surface and some- 

 what convex on the inner. The posterior extremity projects 

 backwards beyond the dental plate a little more than half the 

 length of the latter, and is for the greater part occupied by the 

 glenoid surface, which extends from the upper margin, and is a 

 deep, wide, circular notch, inclining backwards and downwards. 

 In front the symphysial surface is straight, extending the whole 

 depth of the ramus, and is grooved transversely. The dental 

 plate is about two-thirds the entire length of the ramus, and is 

 placed nearer the symphysis than the posterior extremity. The 

 ramus is upwards of 8 inches in length, and- including the thick- 

 ness of the dental plate, is an inch deep." Tig. XX pourtrays 

 the inner surface of the dentary ; the dental plate resting on the 

 upper border ; the symphysis ; and the glenoid notch. Pig. XXI 

 exhibits the other surface with the dental plate overhanging. I 

 may explain that these figures are copies of drawings published 

 by Mr. Atthey. I am not able to give original drawings, as my 

 specimen was destroyed on my travels. Prom the above des- 

 cription it will be seen that each mandible possesses only one 

 tooth, but Mr. T. P. Barkas, in a letter to the " Scientific 



