150 CARBONICOLA, ANTHRACOMYA, AND NATADITES. 
the two valves of any specimen in apposition, which seems to point to the fact 
that this exposure to violence, after the death of the mollusc, caused a separation of 
the two valves of the shell. The beds at Beith seem to have been subjected 
to violence, few perfect specimens being found there, though this locality has 
furnished the bivalve examples figured on Pl. XX, figs. 9 and 10. 
A slighter form of this shell, found in several localities, was mentioned by 
Mr. R. Etheridge, jun., in his paper ‘On our Present Knowledge of the Inverte- 
brate Fauna of the Lower Carboniferous or Calciferous Sandstone Series of the 
Edinburgh neighbourhood, especially of that division known as the Wardie Shales, 
and on the first appearance of certain Species in these Beds” (‘ Quart. Journ. 
Geol. Soc.,’ vol. xxxiv, p. 138, 1878; read November 7th, 1877), under the term 
Myalina crassa var. modioliformis, pointing out that it had been described and 
figured by Captain Thomas Brown ; and I take the figure by Dr. Rhind (‘ Age of the 
Earth,’ pl. 1, fig. 7), the original of which is said to have come from Woodhall, 
Water-of-Leith, to refer to the same form. Indeed, it is very probable that 
Captain Brown reproduced Dr. Rhind’s figure, the two being exactly similar. 
Mr. Etheridge says: ‘‘ As I cannot distinguish any characters, either external or 
internal, by which to specifically distinguish Captain Brown’s shell from Dr. 
Fleming’s, I propose to adopt the name given by the former as a varietal designa- 
tion for the thinner and lighter form of Myalina crassa.” I have not thought it 
necessary to preserve the varietal name, agreeing as I do with the observation 
of Mr. Etheridge that the only difference to be observed is one dependent on 
environment. The shells from the neighbourhood of Edinburgh are undoubtedly 
of much slighter build and somewhat undersized, and they are said to occur with 
Schizodus Salteri above and below a marine shale. 
This stratigraphical arrangement points to changes of conditions which were 
doubtless inimical to the robust growth of the shell, and I find that, judging 
from the specimens I have seen, the shells of Schizodus are also somewhat dwarfed. 
The beds may have occupied a position near the mouth of a great river, where the 
volume of fresh water laden with mud proved hostile to the Mollusca which lived 
in or just beyond the estuary. It should, however, be noted that these slighter 
forms are the older in point of age, the Calciferous-Sandstone Series being older 
than the Carboniferous-Limestone beds of Cults and Beith, and that these forms 
are really the ancestors of those which, under a series of more favourable condi- 
tions, grew so luxuriantly at the two localities just mentioned. 
