206 Dr Hizzerr 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.) Heremarked 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. Acassziz 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 ther teeth. 
While Jabouring under this uncertainty, and in the absence 
of any satisfactory specimen whatever from Burdiehouse, there 
