The _Articulate_ animals are numerously represented in the Cambrian deposits, but exclusively by the class of _Crustaceans_. Some of these are little double-shelled creatures, resembling our living water-fleas (_Ostracoda_). 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 Palæozoic 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.)]