CERATODUS. 31 



The original, from the Rhaetic of Aust Cliff, is now in the Cabinet of the Earl of 

 Enniskillen, whom I have to thank not only for the opportunity of studying this fossil, 

 but for invaluable help of all kinds in the preparation of the present memoir. I regret 

 that I cannot as yet accept Agassiz's decision as to the specific distinctness of C. obtusus, 

 supported as it is by opinions no less worthy of respect. After tracing the remarkable 

 variations of C. polymorphus, I should be slow to rank obtusus as specifically distinct. 

 Still, it cannot be exactly paralleled by any tooth known elsewhere, nor can its gradation 

 into polymorphus be clearly made out as yet. I propose to leave it as a questionable 

 species, waiting for a well-founded decision as to its true place. 



[C. disauris, Ag7\ 



Another form, even more aberrant than C. obtusus, is the single specimen of 

 C. disauris (C. bicornis of Agassiz's plate). Here, again, I quote the original descrip- 

 tion : 



" This is the most singular species of the genus ; it ought even to form a separate 

 group, so much does it differ from its congeners, at least in form. 



" This tooth is also derived from the Lias [Rhaetic] of Aust Cliff, and is contained in 

 the collection of Mr. Johnson at Bristol. What particularly distinguishes it is, that its 

 internal margin is uniformly rounded at both ends, and that the denticles of the external 

 margin, instead of converging regularly towards one of the angles, stand out at right 

 angles to the side, which is itself rectilinear. The surface of the crown is very smooth 

 and slightly uneven (bosselee)." 1 



The obvious difference between this tooth and that of any other known to us does 

 not seem to me to carry the whole question. Equally strange eccentricities affect the 

 dentition of individual hares and camels, and every person with opportunities for observa- 

 tion must have observed distorted teeth in the Salmon and other common fishes. The 

 difference in form between C. disauris and C. parvus or serratus is so extreme, and as 

 one may perhaps say, so purposeless, that I prefer to treat it as an individual and not 

 as a specific trait, due possibly to injury repaired during life. Were the two central 

 denticles of serratus crushed in an early stage of growth and never properly renewed, 

 the resulting tooth would be not unlike disauris. Proof of the specific character of the 

 peculiarities of the supposed species can, as it seems to me, be afforded only by their 

 recurrence in a constant degree. 



1 ' Poissons Fossiles,' t. iii, p. 135. The original specimen is in the cabinet of Sir P. M. de Grey 

 Egerton at Oulton Park. 



