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 Lurypterids of the Upper 
Silurian and Devonian are but feebly represented, and make 
their final exit here from the scene of life. Theirplace, how- 
ever, is taken by peculiar forms belonging to the allied group 
of the Xzphosura, represented at the present day by the King- 
crabs or ‘‘ Horse-shoe Crabs” (Zzmulus). Characteristic forms 
of this group appear in the Coal-measures both of Europe and 
America ; and though constituting three distinct genera (Prest- 
wichia, Belinurus, and Euprodps), they are all nearly related 
to one another. The best known of them, perhaps, is the 
Prestwichia rotundata of Coalbrookdale, here figured (fig. 121). 
The ancient and _ for- 
merly powerful order 
of the Z/rzlobites 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 (PAd/- 
lipsta, Griffithides, and ‘Fig. al Pac ee en n , ud 
Brachymetopus), be- eee Coal-measures, aus (After Henry 
longing to a single fa- 
mily. The Phillipsia seminifera here figured (fig. 122, a) isa 
characteristic species in the Old World. The Water- fleas 
(Ostracoda) 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 Lxtomoconchus Scouleri (fig. 122, ¢), may attain 
a length of from one to three quarters of aninch. The old 
group of the Phyopods 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 Lstherie and their allies, which present a curious 
