estwichia rotundata_, a Limuloid
Crustacean. Coal-measures, Britain. (After Henry Woodward.)]

[Illustration: Fig. 122.--Crustaceans of the Carboniferous Rocks.
a, _Phillipsia seminifera_, of the natural size--Mountain Limestone,
Europe; b, One valve of the shell of _Estheria tenella_, of the
natural size and enlarged--Coal-measures, Europe; c, Bivalved
shell of _Entomoconchus Scouleri_, of the natural size--Mountain
Limestone, Europe; d, _Dithyrocaris Scouleri_, reduced in
size--Mountain Limestone, Ireland; e, _Paloeocaris typus_, slightly
enlarged--Coal-measures, North America; f, _Anthrapaloemon gracilis_,
of the natural size--Coal-measures, North America. (After De
Koninck, M'Coy, Rupert Jones, and Meek and Worthen.)]

The _Crustaceans_ of the Carboniferous rocks are numerous, and
belong partly to structural types with which we are already familiar,
and partly to higher groups which come into existence here for the
first time. The gigantic _Eurypterids_ of the Upper Silurian and
Devonian are but feebly represented, and make their final exit
here from the scene of life. Their place, however, is taken by
peculiar forms belonging to the allied group of the _Xiphosura_,
represented at the present day by the King-crabs or "Horse-shoe
Crabs" (_Limulus_). Characteristic forms of this group appear
in the Coal-measures both of Europe and America; and though
constituting three distinct genera (_Prestwichia, Belinurus_,
and _Euprooeps_), they are all nearly related to one another. The
best known of them, perhaps, is the _Prestwichia rotundala_ of
Coalbrookdale, here figured (fig. 121). The ancient and formerly
powerful order of the _Trilobites_ also undergoes its final
extinction here, not surviving the deposition of the Carboniferous
Limestone series in Europe, but extending its range in America
into the Coal-measures. All the known Carboniferous forms are
small in size and degraded in point of structure, and they are
referable to but three genera (_Phillipsia, Griffithides_, and
_Brachymetopus_), belonging to a single family. The _Phillipsia
seminifera_ here figured (fig. 122, a) is a characteristic species
in the Old World. The Water-fleas (_Ostracoaa_) are extremely
abundant in the Carboniferous rocks, whole strata being often
made up of little else than the little bivalved shells of these
Crustaceans. Many of them are extremely small, averaging about
the size of a millet-seed; but a few forms, such as _Entomoconchus
S