fi'om the Shales of the Northumberland Coal-field. 79 



to the anterior extremity. The whole surface is covered with 

 a coating of enamel, which considerably obscures the mark- 

 ings. On the exposed extremity the enamel is thickened and 

 is very finely granulated, but here it is never perfect, the 

 greater portion of the ornamentation being always exposed ; or, 

 at least, it is so in my specimen. The under surface of the 

 scale is also grooved lengthwise, but with great inequality, 

 and the grooves are so strongly and irregularly pitted that the 

 ridges which bound them are distinct only at the margins. 



On the whole, then, it is evident that the scales of our fish 

 differ considerably from those of Dipterus^ in which they are 

 described as nearly circular when detached ; the sculpture or 

 ornamentation of the surface also seems to vary. There is 

 likewise another and perhaps more important difference found 

 in the dental plates. In Ctenodus these plates are wliat the 

 name implies — solid expansions of dense matter apparently 

 composed of dentine and bone ; and the ridges are equally 

 solid outgrowths of the surface of the plates, bearing tubercles 

 or denticles ; or, in other words, the plates are covered with 

 denticulated ridges. In Dij^terus, on the contrary, the dental 

 organs are uniformly spoken of by Hugh Miller as " patches 

 of palatal teeth "* ; and Agassiz describes the ridges or " ca- 

 rinse " of specimens obtained in the Old Red Sandstone of 

 Russia as " composed of series of imbricated and articulated 

 teeth " t ; and the figm-es representing them entirely confirm 

 the description. In the Ctenodi this arrangement cannot be 

 said to exist, though there *are in a few of the species slight 

 traces of something of the kind, particularly in C. elegans. 



It may, however, be doubted how far such characters should 

 be considered of generic importance. They point out, never- 

 theless, the propriety of retaining for the present, at least, 

 until more is known respecting them, the generic appellation 

 of Ctenodus for our coal-measure Ctenododipterini. 



The body of the Newsham specimen is not well defined ; 

 but as far as the general characters can be determined, they 

 agree with those oi DijMrus. The head is apparently round 

 and short, being about one-fourth the length of the entire fish, 

 which is 3 inches long ; and it exhibits in a distinct manner 

 the underside of the basisphenoid and the praisphenoid bones 

 united together. They assume the form of a rhomboidal plate, 

 with a flattened process extending from the anterior and pos- 

 terior angles ; the bones bearing the dental plates have been 



* Op. cit. 



t Monographie des Poissons Fossiles du Vieux Gres-rouge, troisiemc 

 Im-aisou, p. 123, tab. 33. 



7* 



