2-]^ 



ARACHNIDA — XIPIIOSURA 



appears to be represented in the Mesozoic and Tertiary deposits, 

 but in the Palaeozoic formations (principally in the Upper 

 Silurian, the Old Eed Sandstone, and the Coal Measures) 



several genera have 

 been found, most 

 of which differ from 

 Limuhis in having 

 some or all of the 

 segments of the ab- 

 domen free ; in this 

 respect they re- 

 semble the Euryp- 

 terida, but differ 

 from them in the 

 number of segments. 



Fig. 159. ^A. //ejwifts^M^M/n/^ou^es, Woodw., Upper Silurian, -^ ^ »' 



Leintwardine, Shropshire. Natural size. (After Wood- 159, A), from the 

 ward.) "&. Preshoichia (Euroops) danae{M.^fi'k), Ca.rhom- Oil,,,,,,, fi,„ „„ , 

 ferous, Illinois, x |. (After Packard.) ^'511Ul]an, tnc Seg- 



ments of the ab- 

 domen are divisible into two groups (mesosoma and metasoma) 



in the same way that they are in Eurypterids ; 



the first six segments have broad, short terga, 



the lateral margins of the sixth being divided 



into two lobes, probably indicating the presence 



of two fused segments; the last three segments 



are narrower and longer than the preceding, 



and at the end is a pointed tail -spine. In 



Belinurus (Fig. 160) from the Carboniferous, 



the two regions of the abdomen are much less 



distinct ; there are eight segments, the last 



three of which are fused together, and a long 



tail-spine. In Neolimulus, from the Silurian, 



there seems to be no division of the abdomen 



into two regions, and apparently all the segments were free. 



On the other hand, in Prestwichia (Carboniferous), all the 



segments of the abdomen, of which there appear to be seven 



only, were fused together (Fig. 159, B). 



In the Palaeozoic genera the median or axial part of the 



dorsal surface is raised and distinctly limited on each side, so 



presenting a trilobed appearance similar to that of Trilobites. 



Fig. 160. — Belinurus 

 reginae, Baily, Coal 

 Measures, Queen's 

 Co., Ireland, x 1. 

 (After Woodward). 



