Nevertheless, there
is an equally decided and striking amount of difference between
these successive faunas, due to the fact that the great majority
of the Carboniferous _species_ are new; whilst some of the most
characteristic Devonian _genera_ have nearly or quite disappeared,
and several new genera now make their appearance for the first
time. Thus, the characteristic Devonian types _Heliophyllum,
Pachyphyllum, Chonophyllum, Acervularia, Spongophyllum, Smithia,
Endophyllum_, and _Cystiphyllum_, have now disappeared; and the
great masses of _Favosites_ which are such a striking feature
in the Devonian limestones, are represented but by one or two
degenerate and puny successors. On the other hand, we meet in
the Carboniferous rocks not only with entirely new genera--such
as _Axophyllum, Lophophyllum_, and _Londsdaleia_--but we have an
enormous expansion of certain types which had just begun to exist
in the preceding period. This is especially well seen in the Case
of the genus _Lithostrotion_ (fig. 116, b), which more than any
other may be considered as the predominant Carboniferous group of
Corals. All the species of _Lithostrotion_ are compound, consisting
either of bundles of loosely-approximated cylindrical stems, or of
similar "coral-lites" closely aggregated together into astraeiform
colonies, and rendered polygonal by mutual pressure.
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