84 HISTORICAL PALEONTOLOGY. 



fish, whilst some of them have certainly nothing whatever to 

 do with the worms. Lastly, the Cambrian beds often show 

 twining cylindrical bodies, commonly more or less matted 

 together, and not confined to the surfaces of the strata, but 

 passing through them. These have often been regarded as 

 the remains of sea-weeds, but it is more probable that they 

 represent casts of the underground burrows of worms of simi- 

 lar habits to the common lob-worm (Arenicola) of the present 

 day. 



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 (Pliyllopoda}. 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 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 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. 



The upper surfaces 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 



