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.  The first and most anterior of these divisions is a shield or buckler which covers the head; the second or middle portion is composed of movable rings covering the trunk ("thorax “); and the third is a shield which covers the tailor “abdomen.”  The head-shield (fig. 31, e) is generally more or less semicircular in shape; and its central portion, covering the stomach of the animal, is usually strongly elevated, and generally marked by lateral furrows.  A little on each side of the head are placed the eyes, which are generally crescentic in shape, and resemble the eyes of insects and many existing Crustaceans in being “compound,” or made up of numerous simple eyes aggregated together.  So excellent is the state of preservation of many specimens of Trilobites, that the numerous individual lenses of the eyes have been uninjured, and as many as four hundred have been counted in each eye of some forms.