C.—GEOLOGY. 107 
The species-group of Hncrinurus sexcostatus would appear to show 
a similar evolutional stage at similar horizons, though in this the 
line at present is incomplete. 
Facts such as I have enumerated in the different groups of fossils 
with which we have mainly to deal in Lower Paleozoic rocks show 
that, if viewed from the evolutional standpoint, even the most meagre 
fauna may yet in many cases be made to yield a considerable amount 
of information as to the age of the beds containing it, for the evolu- 
tional succession, once established, can be applied anywhere, and the 
explanation of any apparent anomalies will be more correctly sought 
in the mutual relations of the rocks than in the faunas they contain. 
Moreover, as regards the varying shallow-water faunas, even those 
which have a generally similar aspect may be shown definitely to be 
of different ages when one as a whole contains fossils at a different 
stage of evolution from the other, and the apparent similarity, so 
striking upon superficial examination, will then be regarded as deter- 
mined by physical conditions and not by contemporaneity. Working 
on lines such as these we shall be enabled to visualise more definitely 
the conditions which governed the distribution of the different faunas 
in the remote past, and thereby acquire a more accurate conception of 
the changes in physical geography that must have taken place with the 
progress of time. 
