292 ARACHNIDA—EURYPTERIDA CHAP. 
from the mesosoma to the metasoma; but in some others, 
which in this respect are less primitive, such as Slimonia 
(Fig. 165), the posterior five segments of the body (like those 
of Scorpions) are distinctly narrower and longer than the preced- 
ing segments. The long tail-spine of Hurypterus is represented 
in Slimonia by an oval plate produced into a spine at the end 
(Fig. 165,19); whilst in some species of Pterygotus the plate 
is bilobed at the posterior end (Fig. 164, 19). In Hughmilleria 
the tail-spine is lanceolate. 
The Eurypterids resemble the Xiphosura in many respects. In 
both groups the prosoma consists of at least six fused segments, 
and bears two pairs of eyes—one pair simple, the other grouped 
eyes—on the dorsal surface of the carapace. The number and 
position of the appendages of the prosoma in Eurypterids agree 
with those of Limulus. The chelicerae are closely similar in both 
cases. The coxae of all five pairs of legs in Eurypterids are 
toothed and function in mastication ; similarly in Limulus all are 
spiny except the coxae of the last pair of legs. In both a similar 
epicoxite 1s present on the coxae. The number of joints in the 
legs is somewhat greater in the Eurypterids than in Limulus, 
and in the former none of the legs end in chelae, whereas in the 
latter all the walking legs, except the last, and also the first in 
the male, may be chelate. The metastoma of Eurypterids differs 
in being a large unpaired plate, but is represented in Limulus by 
the pair of relatively small chilaria. On the mesosoma the 
genital operculum and plate-like appendages with branchial 
lamellae are similar in both groups, but in the Eurypterids 
the genital operculum shows a greater development and covers 
the second segment, which is without plate-like appendages. A 
striking difference between the two groups is seen in the seg- 
ments of the mesosoma and metasoma; in Eurypterids these are 
all free, whilst in Limulus they are fused together, but this 
difference is bridged over by some of the Palaeozoic Xiphosura 
(Fig. 159, A) in which those segments are free. 
The Eurypterids present a striking resemblance to Scorpions. 
In both groups the segments in the three regions of the body are 
the same in number, and the appendages of the prosoma also 
agree in number and position. The pre-oral appendages are 
chelate in both, but the second pair of appendages are chelate in 
the Scorpions only. In Eurypterids the coxae of the five pairs 
