84 HISTORICAL PAL/EON TOLOGY. 



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 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 (Limulus], and the Isopods 

 ("Slaters," Wood-lice, &c.) Indeed, one member of the last- 

 mentioned order, namely, the Scrolls 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 



