534 
THE GEOLOGIST. 
inferred from other data — perhaps chiefly from the nature of the animals of the j 
coal-measures. I uill only say that there are in the neighbourhood of Man- J 
Chester instances of thick' beds of coal, overlaid immediately by uniform fine 
shale (without a particle of grit or sand which might indicate a subsidence of I 
the surface), whick shales contain only marine shells. The majority of the 
shells, however, of the middle and upper coal-measures are not so decisive of 
their habitat, and I wish now to call attention to some of these. 
Fig. . 
The so-called " t'/z/o" bands of the coal-measiu'es are so well known as not" 
to need dc scription. In the condition of broad banks often several feet thick, tliey 
occur throughout all our fields : and though rare in the lower measures, they 
are not absent from them. A number of species have been described, some 
of them probably the merest varieties of the three or foiu- well-marked forms 
originally described by So wcrby in the "Mineral Conchology." But others 
are distinct forms, and some yet remain to be distinguished and added to our 
lists. Some species appear to be characteristic of particular seams or bands 
in the coal, and to be almost confined to these; others, as the common TJnio 
acid US, arc found even so far down as tlie mountain limestone shale, and lived 
on to the close of the coal period with a wide geographical range. 
Tlicy have been distinguished from the U/iio of oiu- freshwater lakes and 
rivers by Prof. King, and tliough it is as yet by no means certain that they are 
allied to the Mi/adce, the chai*acter of their epidermis, and the internal position 
of the ligament that binds the shells together, leads to the belief that they 
were marine shells. 
J/it/tracosia.— They arc tliick shells, and yet destitute of hinge-teeth. The 
