84 HISTORICAL PALZONTOLOGY. 
these is the Wymenocaris vermicauda of the Lingula Flags (fig. 
32, @). By far the larger number of the Cambrian Crustacea 
belong, however, to the remarkable and wholly extinct group 
of the Zyrzlobites. 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 Paleozoic epoch, and we have no Crusta- 
ceans 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 (Zzmulus), and the Isopods 
(‘‘Slaters,” Wood-lice, &c.) Indeed, one member of the last- 
mentioned order, namely, the Sevo/zs 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. 
The upper surface of the body of a Trilobite was defended 
by a strong shell or ‘‘ crust,” partly horny and partly calcare- 
ous 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 mov- 
able rings covering the trunk (“thorax”); and the third is a 
shield which covers the tail or “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 usu- 
ally 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. ‘The eyes may be sup- 
ported upon prominences, but they are never carried on mov- 
able stalks (as they are in the existing lobsters and crabs) ; and 
