in the Neighbourhood of Edinburgh. 271 
As for the remains of Cestraciontes, (and perhaps of the Me- 
galichthys,) which appear in more than one description of car- 
boniferous limestones, they point to estuaries, no less than to 
fresh-water lakes, as having been in primeval times frequented 
by large animals in quest of their prey. 
The foregomg researches shew,— 
lst, That it is by the presence or absence of acknowledged 
pelagic mollusca, corallines, &c. that indications are afforded of 
the great difference between fresh-water and marine deposits. 
2dly, That if, along with marme mollusca or corallines, we 
find the plants of coal-fields in a quantity comparatively small, an 
estuarian limestone may be inferred. And, 
3dly, That if marine mollusca or corallines should be entirely 
absent ma limestone, and if plants should be abundantly found in 
it, an indication would be afforded of a calcareous deposit which 
took place amidst the fresh-water rivers or lakes of primeval 
marshes; which indication would be still more favoured, if we 
should find, in addition, recognised genera of fresh-water shells, 
the entomostraca of stagnant marshes, or the fish incidental to 
coal-fields. 
It is however admitted, that the presence of fish is an ambi- 
guous criterion. The smaller fish of lakes, or rivers, are known in 
recent times to venture into estuaries, while larger fish enter fresh- 
water rivers, or lakes, im pursuit of their prey. 
From these conclusions it is evident, that while a broad line of 
demarcation subsists between marine and fresh-water limestones, 
it is by no means impossible that limestones of an estuarian and 
fluviatile formation may, in some cases, be more diffieult to dis- 
tinguish, and particularly, when it is kept in view, that, from a 
remote period of the globe, communications between rivers and 
seas must have subsisted, as at the present time. Thus, during 
the actual state of our globe, the Ganges, as we approach close 
