ARACHNIDA. EURYPTERIDA 343 



Distribution of the Xiphosura 



Fossil Xiphosura are rare, except in the Solenhofen 

 Limestone (Upper Jurassic). The chief genera are : 



Silurian 1 . Hemiaspis, Neolimulus, Bunodes, Pseudoniscus. 

 Devonian. Protolimulus, Prestwichia. 

 Carboniferous. Belinurus, Prestwichia. 

 Permian. Prestwichia in North America. 

 Trias to Oligocene and Recent. Limidus. 



ORDER II. EURYPTERIDA 



The Eurypterids are found only in the Palaeozoic rocks 

 and are remarkable for the large size which they often 

 attain ; one form (Pterygotus anglicus) reaches a length of 

 six feet and is the largest Arthropod known. The Eury- 

 pterids have a scorpion-like appearance ; but, unlike the 

 scorpions, they were all aquatic animals, and, with the 

 possible exception of forms found in the Coal Measures, 

 all marine. The body is compressed dorso-ventrally, and 

 is protected by a chitinous exoskeleton (fig. 147) which is 

 covered with small scale-like markings. 



The prosoma or cephalothorax consists of the six 

 anterior segments fused together, and is usually quadrate 

 or semicircular in outline. The carapace, which covers 

 the dorsal surface of the prosoma, bears a pair of small, 

 simple eyes near its centre (fig. 147, e), and a pair of large, 

 lateral eyes — one at each of the outer front margins or at 

 some little distance from those margins (d). 



1 Aglaspis, from the Cambrian of Wisconsin, is perhaps referable to 

 the Xiphosura. 



