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.
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