88 GUIDE TO THE FOSSIL INVEETEBEATE ANIMALS. 



Gallery paired plate with a median process, the whole known 

 as the genital operculum. This appears to correspond to 

 two segments. The four following segments bear each a 

 somewhat similar plate, to which were probably attached 

 leaf-like gills. The six remaining abdominal segments have 

 no appendages, except the telson. The surface of the 

 chitinous envelope usually bears a scale-like ornament. 

 The Eurypterida are first found in Ordovician rocks, and 

 attained their maximum in both numbers and size about the 

 beginning of the Devonian Epoch, when they seem to have 

 frequented shallow waters and lagoons ; they are found in 

 the Coal Measures under circumstances indicating a brackish 

 or freshwater habitat ; the last survivor is associated with 

 land-plants of Permian age. The British fossils belong 

 chiefly to the genera Euryioterus, Slimonia, and Pterygotiis. 

 The remains of the last-mentioned, from the Old Eed 

 Sandstone of Scotland, are large and conspicuous objects, 

 widely known through the writings of Hugh Miller. In 

 the Silurian rocks of Oesel in the Baltic smaller species 

 of Eurypterus and Pterygotus occur in a beautiful state of 

 preservation. The great Stylonurus and the smaller Hugh- 

 milleria lived in North America during the Devonian Epoch. 

 Table-case Next come fossils of the Order Xiphosura (sword-tails), 

 WaU^case which Limulus, the king-crab, is the living representative 

 13c. (^'^to- ^^2). Here the fore-part of the body is proportionately 

 much larger, and is covered by a domed shield of horse-shoe 

 outline. Near its middle line is the pair of ocelli, and 

 further back on each side, about halfway from the margin, 

 is a compound eye. The hind part of the body is, in 

 Limulus, covered by 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 perhaps 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 surrounded by appendages, which scarcely 

 differ from those of Eurypterida beyond the removal of the 

 sixth pair from a share in biting the food. The six segments 

 of the hind-part carry paired plate-like appendages, as in 

 Eurypterida, the first forming the genital operculum, the rest 

 bearing gills on their hinder surfaces. Limulus then differs 

 from the Eurypterida mainly in the fusion of the abdominal 

 segments and their reduction from twelve to six. In the 

 very young Limulus, however, there are nine such segments, 



