2/8 



ARACIINIDA XIPHOSURA 



C11A1 1 . 



rs 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 

 Limulus 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. 

 In Hemiaspis (Fig. 



FIG. 159. A. Hemias2ns hmuluiJes, \\oothv., Upper &ihman. ^ 



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



ward.) B. Prestwichia (Euroops) danue (Meek), Carboni- gi|vm'i n f^g S6'- 

 ferous, Illinois, x f. (After Packard.) 



ments of the ab- 

 domen are divisible into two groups (mesosoma and rnetasoma) 

 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. 



li 



FIG. 160. lie/ hum's 

 reginae, Baily, Coal 

 Measures, Queen's 

 Co., Ireland, x 1. 

 (After Woodward). 



