26 
LEA ON SOME NEW FOSSIL MOLLUSCS, &c. 
( Posidonomya) a range from the lower Silurian to the upper portion of the Oolite, its 
maximum existence being in the Devonian. 
The importance of the existence of marine shells in the shales enclosing a seam of 
coal, will be admitted at once, when we reflect on the necessity of the fact that it 
designates a return of the ocean to a point from which it had receded; and were it 
supposed to be a fact that all shales were deposited by marine action, the return of 
the ocean in some carboniferous deposits, would require an oscillation so frequent, as 
to forbid such a theory. In Belgium, where it is said there are one hundred and 
twenty-six seams of coal, in successive superposition, if each deposit of Carbon had 
an inferior and a superior slate of this marine origin, the salt water would be required 
to advance and recede two hundred and fifty-two different times. This frequent 
oscillation of the surface, from a position sufficiently elevated above the level of the 
sea to produce air plants, to a submergence sufficiently depressed to sustain marine 
life, would require a frequency and regularity of oscillation that we could not 
reasonably admit. That depressions and elevations occurred we cannot doubt, and 
that some elevations were of long continuance is certain, for we have the existence of 
large fossil trees still erect, in a vertical position, and of great magnitude. Such have 
been observed in France, in England, and in Nova Scotia. These are not marine 
plants, but great trees, and chiefly Palms and gigantic Ferns, requiring a warm and 
humid atmosphere to sustain their rapid and great growth. 
It is evident, from the horizontality and parallelism of the beds of Carboniferous 
strata, that they were deposited at a tranquil period. Independently of the general 
even condition of the seams of coal, the perfection of the delicate fossil flora, existing 
in the shales, would bear ample testimony as to the fact. It must I think, therefore, 
be admitted, that the enclosing slates, in which we find these numerous coal plants, 
were terrestrial, and not submarine. The exception is when marine molluscs are 
found, which must be at points where there is no floral exhibition. In these cases 
we naturally look for Ichythic existence also, and this view is sustained by the fossil 
remains of impressions of scales, and, in some cases, whole fishes, with their 
characteristic heterocercal tails. 
The genus Modiola came into existence during the epoch of the Upper Silurian, and 
has continued through the various strata to the present time, where it has obtained 
its maximum. It is now a very extensive form. The vastly long period that this 
genus has survived the great changes on the surface of the globe, clothes it with more 
than usual interest. Mr. Morris, in his “Catalogue of British Fossils,” mentions 
forty-four species in the strata of England and Ireland, one only of which existed 
during the deposit of the Coal Measures. Ten are found below, six of them being in 
the Carboniferous Limestone, immediately below, and four in the Silurian. The 
Modiola carinata, Sow.,* is found in the Coal Measures of Coalbrookdale. Bronn, in 
* Geological Transactions, 2d ser., vol. 5, pi. 39, fig. 15. 
