THE CARBONIFEROUS PERIOD. 



183 



Fig. 121 . Prestwichia rotundata, a Llmuloid 

 Crustacean. Coal-measures, Britain. (After Henry 

 Woodward.) 



Prestwichia rotundata of Coalbrookdale, here figured (fig. 121). 

 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 

 Europe, but extending 

 its range in America 

 into the Coal-meas- 

 ures. All the known 

 Carboniferous forms 

 are small in size and 

 degraded in point of 

 structure, and they 

 are referable to but 

 three genera {Phil- 

 lipsia, GriffitJiides, and 

 Brachymetopus}, be- 

 longing to a single family. The Phillipsia seminifera here 

 figured (fig. 122, a,) is a characteristic species in the old World. 

 The water-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 extremely small, averaging about the size of a 

 millet-seed ; but a few forms, such as Entomoconchus Scouleri 

 (fig. 122, <r), may attain a length of from one to three quarters 

 of an inch. The old group of the Phyjllopods is likewise still 

 represented in some abundance, partly by tailed forms of a 

 shrimp-like appearance, such as Dithyrocaris (fig. 122, d), and 

 partly by the curious striated Estherice and their allies, which 

 present a curious resemblance to the true Bivalve Molluscs (fig. 

 122, &). Lastly, we meet for the first time in the Carboniferous 

 rocks with the remains of the highest of all the groups of 

 Crustaceans namely, the so-called " Decapods, " in which there 

 are five pairs of walking-limbs, and the hinder end of the body 

 ("abdomen") is composed of separate rings, whilst the anterior 

 end is covered by a head-shield or " carapace. " All the Carbon- 

 iferous Decapods hitherto discovered resemble the existing 

 Lobsters, Prawns, and Shrimps (the Macrura), in having a long 

 and well-developed abdomen terminated by an expanded tail-fin. 



