88 GUIDE TO THE FOSSIL INVIHiTEBKATE ANLMALS. 
Gallery 
VIII. 
Table-case 
23. 
Wall-case 
13c. 
ill size in diil'ereiit genera, but always ending in pincer- 
claws (chelae). At the sides of and behind the mouth are 
five pairs of limbs (n-vi), variously modified in different 
genera for crawling, swimming, or grasping, but always 
agreeing in having the ba.sal segment toothed to serve as a 
jaw, and those of tlie liindmost pair much larger than the 
others. On the under surface of the first (or second) segment 
of the mid-body are the openings of the reproductive glands, 
covered by a paired plate with a median process, the whole 
known as the genital operculum. Tlie four following seg- 
ments bear each a soniewliat similar plate, to w'hich w'ere 
probably attached leaf-like gills. The segments of the hind- 
body have no appendages, except the telson. The surface of 
tlie chitinous envelo]>e usually bears a scale-like ornament. 
Tlie Eurypterida are first knowm from Cambrian rocks, and 
attained their maximum in both numbers and size about the 
beginning of the Devonian Epoch, wdien they seem to have 
frequented shallow^ waters and lagoons ; they are found in 
the Coal Measures in circumstances indicating a brackish 
or freslnvater habitat ; the last survivor is associated wdth 
land-plants of Permian age. The Pritish fossils belong 
chiefly to the genera EniYptems, Slimonia, and Flerygotus. 
The remains of the last-mentioned, from the Old Ked 
Sandstone of Scotland, are large and conspicuous objects, 
widely knowm through the writings of Hugh IMiller. In 
the Silurian rocks of Oesel in the Baltic smaller species 
of Euryptcriis (Fig. 41) and Ptcryyotus occur in a beautiful 
state of preservation. The great Stylonurns and the smaller 
Huyhmilleria lived in the Devonian seas of North America. 
Next come fossils of the Order Xiphosura (sword-tails i, 
of which Litmdm, the king-crab, is the living representative 
(Fig. 42). Here the fore-body is pro])ortionately much 
larger, and covered by a domed shield of horse-shoe outline. 
Near its middle line are the ocelli, and further back on 
oacli side, aljout halfway from tlie margin, is a compound 
eye. The mid- and hind-bodies are, in Limulus, covered liy a 
single shield, with six spines at each side and with grooves 
on its back indicating that it is composed of certainly six 
segments and jirobably more. This is separated from the 
front shield by a strongly marked flexible articulation, and 
the bayonet-shaped telson is jointed to it behind. The 
under-surface of the fore-part has a central mouth sur- 
rounded by appendages, which scarcely differ from those 
of Eurypterida beyond the removal of the sixth pair from 
