THE CARBONIFEROUS PERIOD. 



179 



and belong partly to structural types with which we are already 

 familiar, and partly to higher groups which come into existence 

 here for the first time. The gigantic Eurypterids of the Upper 

 Silurian and Devonian are but feebly represented, and make 

 their final exit here from the scene of life. Their place, how- 

 ever, is taken by peculiar forms belonging to the allied group 

 of the Xiphosura, represented at the present day by the King- 

 crabs or " Horse-shoe Crabs " (Limulus). Characteristic forms 

 of this group appear in the Coal-measures both of Europe and 

 America ; and though constituting three distinct genera (Prest- 

 wic/iia, BelinuruS) and Euproops\ they are all nearly related 

 to one another. The best known of them, perhaps, is the 

 Prestwichia rotundata of Coalbrookdale, here figured (fig. 

 The ancient and for- 

 merly powerful order 

 of the Trilobites also 

 undergoes its final ex- 

 tinction here, not sur- 

 viving the deposition 

 of the Carboniferous 

 Limestone series in Eu- 

 rope, but extending its 

 range in America into 

 the Coal-measures. All 

 the known Carbonifer- 

 ous forms are small in 

 size and degraded in 

 point of structure, and 

 they are referable to 

 but three genera (Phil- 



UpSia, GriffithideS, and Fig. r-z-i.Prestwchia rotundata, a Lirr.nloid 



Brachymetopus], be- wSSSScL) Coal - measures ' Britain ' < A ^ Henry 

 longing to a single fa- 

 mily. The Phillipsia seminifera here figured (fig. 122, a) is a 

 characteristic species in the Old World. The AVater- fleas 

 (Ostracoaa) are extremely abundant in the Carboniferous rocks, 

 whole strata being often made up of little else than the little 

 bivalved shells of these Crustaceans. Many of them are ex- 

 tremely small, averaging about the size of a millet-seed ; but a 

 few forms, such as Entomoconchus Scouleri (fig. 122, c), may attain 

 a length of from one to three quarters of an inch. The old 

 group of the Phyllopods is likewise still represented in some 

 abundance, partly by tailed forms of a shrimp-like appearance, 

 such as Dithyrocaris (fig. 122, </), and partly by the curious 

 striated Estheria and their allies, which present a curious 



