2G ARACHNIDA—XIPHOSURA CHAP. 
appears to be represented in the Mesozoic and Tertiary deposits, 
but in the Palaeozoic formations (principally in the Upper 
Silurian, the Old Red Sandstone, and the Coal Measures) 
os pes. several genera have 
a 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. 
Fic. 159.—A. Hemiaspis limuloides, Woodw., Upper Silurian, In Hemiasp is (Fig. 
Leintwardine, Shropshire. Natural size. (After Wood- 159, A), from the 
ward.) B. Prestwichia (Euroips) danae (Meek), Carboni- @77,,,.7, ng 
ferous, Illinois, x 3. (After Packard.) Silurian, the ses 
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 ee Send 
three of which are fused together, and a long — Measures, Queen's 
: ; “ 5 ae > Co., Ireland, x 1. 
tail-spine. In Meolimulus, from the Silurian, © (after Woodward). 
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. 
