56 MESSRS. HANCOCK AND ATTHEY ON 



very finely denticulated and curved towards the centre of the 

 rounded extremity, where they become confused and irregularly 

 nodose. Thus in the centre of the exposed portion there is a 

 sort of rosette, which is defined posteriorly by incomplete con- 

 centric wrinkles ; a few similar wrinkles or marks of growth ex- 

 tend 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 our 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 Dijoterus, in which they are de- 

 scribed as nearly circular when detached ; the sculpture or orna- 

 mentation of the surface also seems to vary. There is likewise 

 another and perhaps more important diff'erence found in the den- 

 tal plates. In Ctenodus these plates are what the name implies 

 — solid expansions of dense matter apparently composed of den- 

 tine 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 

 Diiyterus, on the contrary, the dental organs are uniformly spoken 

 of by Hugh Miller as "patches of palatal teeth;"* and Agassiz 

 describes the ridges or "caringe" of specimens obtained in the 

 Old Eed Sandstone of Eussia as "composed of series of imbrica- 

 ted and articulated teeth ; " f and the figures representing them 

 entirely confirm the description. In the Ctcnodi this arrange- 

 ment 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 



* Op. cit. 



t " Monographie des Poissons Fossiles du Vieux Gres-rouge," troisieme livraison, \i. 123, 

 Tab. XXIII. 



