206 Dr Hibbert on the Limestone of Burdiehouse, 



preserved in the British Museum, the latter exhibited teeth of 

 the same size with such as are represented in Plate IX. fig. 6 and 

 7. They had also the same external structure ; they had large 

 plicae at their base; while the remainder of their surface was 

 otherwise smooth. Some of these teeth, it is added, were sharp 

 at one edge ; others were sharp at both edges ; while a third de- 

 scription had sharp and projecting edges at their extremity only. 

 As for their interior structure, there was, in all, a conical cavity, 

 more or less elongated, like that of Fig. 7, and it was in the bot- 

 tom of this cavity that the new teeth were developed upon the 

 old ones falling away. 



The Alternation of Large and Small Teeth. — This is the next 

 circumstance to be investigated. 



When M. Agassiz visited Edinburgh, he had little know- 

 ledge of the distribution of large and small teeth in the Mega- 

 lichthys, except what was derived from the minute specimen of 

 the jaw of one of the fry of the animal, to which I have already 

 adverted. ( See Plate IX. fig. 1 .) He remarked an evident alterna- 

 tion of larger and smaller teeth, yet it was conceived (as far as 

 such a minute and indistinct specimen could countenance any 

 inference whatever) that there was a greater proportion of large 

 teeth, and that canine teeth were placed upon the whole of the 

 jaw. This notion was again countenanced by the large teeth dis- 

 covered in an unconnected state, which had been hitherto found 

 more abundantly than smaller ones. An inspection of the Leeds 

 specimen, however, was unfavourable to this notion, inasmuch as 

 there appeared in this instance to be a far greater proportion of 

 small teeth, while the larger canine teeth were chiefly developed 

 in the fore part of the jaw ; and hence M. Agassiz at first con- 

 ceived, that there might be two genera of sauroid animals allied 

 to each other, but differing in the comparative size, number, and 

 distribution of their teeth. 



While labouring under this uncertainty, and in the absence 

 of any satisfactory specimen whatever from Burdiehouse, there 



