358 Proceedings of the Royal Physical Society. 
kind of guide to the bathymetrical condition under which 
some of the commoner forms of Carboniferous Lamellibranchs 
may have lived. 
The evidence is of this kind:—It is found by observation 
in the field that certain forms of these Mollusca occur 
either generally or else exclusively in strata of a particular 
lithological type. Certain species, for example, occur ex- 
clusively in pure limestones; others are restricted to 
argillaceous limestones and their associated calcareous shales ; 
others again are found only in sandy shales; while yet a few 
others occur generally in sandstones. Those that occur in 
the one kind of deposit are rarely found to range into any 
one of the others. If we assume, as we appear to be 
justified in doing, that the sandstones represent the shallower 
water deposits formed nearest to the land, and that the 
limestones represent the strata formed farthest from the land, 
and, in general, in deeper water, then it follows that the forms 
of molluscan life occurring exclusively in pure limestones are 
those which lived at the greatest depths, those restricted 
to argillaceous and impure limestones and to the associated _ 
calcareous shales are such as lived at lesser depths, while the 
forms restricted to the arenaceous strata would appear to be 
safely regarded as comparatively shallow-water forms. 
Taking the limestone forms first, we may note that 
Conocardium, Cardiomorpha, Macrodon, Entolium, and a 
small number of other Lamellibranch genera, are most 
commonly, or else exclusively, found in limestone. Then, 
in both impure limestones and calcareous shales occur 
Sanguinolites, Allorisma, Edmondia, Lithodomus dactyloides, 
Pinna flabelliformis and its allies, “ Jnoceramus,” and some 
species of Myalina. In calcareous shales occur the great 
majority of the Carboniferous Lamellibranchs, including almost 
all the numerous species of <Aviculopecten, Anthracoptera, 
Anthracomya, Pteronites, Myalina, Nucula and its sub-genera, 
Schizodus, Pleurophorus, Cypricardella, Leptodomus, Sedgwickia, 
and numerous others. Comparatively few species occur 
‘ commonly in sandy shale, and this habitat seems to be 
almost peculiar to Anthracosia, which is almost certainly an 
estuarine and not a marine form, 
