oda_). A few are larger forms, and
belong to the same group as the existing brine-shrimps and
fairy-shrimps (_Phyllopoda_). One of the most characteristic of
these is the _Hymenocaris vermicauda_ of the Lingula Flags (fig.
32, d). By far the larger number of the Cambrian _Crustacea_
belong, however, to the remarkable and wholly extinct group of
the _Trilobites_. These extraordinary animals must have literally
swarmed in the seas of the later portion of this and the whole of
the succeeding period; and they survived in greatly diminished
numbers till the earlier portion of the Carboniferous period.
They died out, however, wholly before the close of the Palaeozoic
epoch, and we have no Crustaceans at the present day which can be
considered as their direct representatives. They have, however,
relationships of a more or less intimate character with the existing
groups of the Phyllopods, the King-crabs (_Limulus_), and the
Isopods ("Slaters," Wood-lice, &c.) Indeed, one member of the
last-mentioned order, namely, the _Serolis_ of the coasts of
Patagonia, has been regarded as the nearest living ally of the
Trilobites. Be this as it may, the Trilobites possessed a skeleton
which, though capable of undergoing almost endless variations,
was wonderfully constant in its pattern of structure, and we
may briefly describe here the chief features of this.

[Illustration: Fig. 31.--Cambrian Trilobites: a, _Paradoxides
Bohemicus_, reduced in size; b, _Ellipsocephalus Hoffi_; c, _Sao
hirsuta_; d, _Conocorypke Sultzeri_ (all the above, together with
fig. g, are from the Upper Cambrian or "Primordial Zone" of
Bohemia); e, Head-shield of _Dikellocephalus Celticus_, from the
Lingula Flags of Wales; f, Head-shield of _Conocoryphe Matthewi_,
from the Upper Cambrian (Acadian Group) of New Brunswick; g,
_Agnostus rex_, Bohemia; h, Tail-shield of _Dikellocephalus
Minnesotensis_, from the Upper Cambrian (Potsdam Sandstone) of
Minnesota. (After Barrande, Dawson, Salter, and Dale Owen.)]

The upper surface of the body of a Trilobite was defended by a
strong shell or "crust," partly horny and partly calcareous in
its composition. This shell (fig. 31) generally exhibits a very
distinct "trilobation" or division into three longitudinal lobes,
one central and two lateral. It also exhibits a more important and
more fundamental division into three transverse portions, which
are so loosely connected with one another as very commonly to be
found separate