in the Neighbourhood of Edinburgh. 261 
with those belonging to an upper stratum. Thus, for instance, 
the Burdiehouse limestone has a character essentially different 
from that of Wardie, which forms a higher stratum. 
It is easy to suppose that, in the change which would cause 
a fresh-water lake to sink beneath the depths of the ocean, and, 
vice versa, to emerge and resume its fluviatile character, many 
races of animals, limited to definite conditions of water, must 
have been destroyed. But it would also appear, that their place 
has been supplied by successions of new races. 
In the next place, a state of the earth’s surface, subject to 
oscillatory movements, may afford an argument relative to ano- 
ther question which has been started. It has been supposed, 
from certain a priori reasonings, that during the carboniferous 
epoch, the air was so filled with carbonic acid gas as to be un- 
favourable to the respiration of animals possessing lungs. And 
hence it has been maintained, that no saurian animal during this 
period could subsist. If, however, the Megalichthys is really al- 
hed to the Lepidosteus, the researches of M. Acass1z have shewn 
that recent sauroid fish possess not only lungs, but likewise a 
trachea, and a glottis. The argument, therefore, of the air having 
been so filled with carbonic acid gas as to be fatal to the respira- 
tion of reptiles which possess lungs, becomes at least suspicious. 
The question, therefore, remaining is to the following ef- 
fect: If the Megalichthys, in common with the Lepidosteus, has 
possessed lungs, to what condition of the earth’s surface might 
these lungs bear reference? The solution of this question can 
perhaps only meet with a satisfactory solution by an appeal to 
the actual habits of the recent Lepidosteus, which dwells among 
the large lakes of South America, but of which I can find no 
written account whatever. 
In the absence of this information, it may perhaps be allow- 
able to throw out a conjecture with regard to the oscillatory 
movements, which were going on upon the earth’s surface during 
