29G ON FOSSIL FISH-TEETH. 



From the examination of all the specimens to which he had access, 

 Agassiz arrived at the conclusion that, in all cases, the comparatively 

 smooth, or unhorned side of the teeth was that which was internal in the 

 jaw, the horns or projecting ridges being external ; and that these teeth 

 were not arranged in more rows than one in the mouth of the fish, but 

 that in each side, of the upper and of the lower jaws, one tooth only 

 was fixed making four teeth in all in the mouth. He concluded also that 

 the straighter portion of the smooth side of the tooth was anterior. .Tliis 

 being granted, it followed that all the horns of the tooth would be point- 

 ed forward, and those which were, in this case, the posterior horns were 

 also the larger, and more marked. Agassiz also pointed out two 

 distinct groups of these teeth, one flat and broad, the other comparatively 

 elongated with more elevated ridges or horns. He alludes to the possi- 

 bility of one of these varieties belonging to the upper jaw, and the other 

 to the lower, but without fixing the point. 



He considered the fish, to which these teeth belonged to have been 

 more allied to the Cestraciont group, than to any other. Subsequent ob- 

 servers, in adding somewhat to the knowledge of this remarkable genus, 

 have referred it to a different position. Bkonn places Ceratodus amonf 

 the Chimeridae, as does also Plieninger, who has described several species 

 from the Muschelkalk and the Keuper. A few other species or varieties 

 have been figured in difterent works, but little has been really added to 

 our knowledge of the genus. Quenstedt* has recently described another 

 variety (^Ceratodus Cloacinus) from the beds below the lias, (" the forerun- 

 ners of the lias"). This was also nearly the geological position of all 

 previously known species. Only one species has been found so high as 

 the Stonesfield slate {^Ceratodus Phillips ii) all the others being from beds 

 lower in the series, and ranging from the upper " Bunter Sandstein" of 

 the vicinity of Magdeburg to the boundary breccia between the Keuper 

 and the lias. 



* Der Jura, 1S5S, p. 34. 



