XI EXTERNAL FEATURES 
structure, in the genus Pterygotus (Fig. 164, 5) they agree closely 
with the second, third, and fourth pairs, 
and in Slimonia (Fig. 165, 5) they are 
nearly the same as the third and fourth 
pairs. The sixth pair of appendages are 
much larger and more powerful than the 
fifth pair in nearly all genera; in Stylon- 
urus (Fig. 166), however, the sixth pair 
are similar to the fifth, both being greatly 
elongated and slender; also in Husarcus 
(Drepanopterus) the sixth pair differ less 
from the preceding pair of appendages 
than is usually the case. 
In Pterygotus there is a well-developed 
epistome (Fig. 164, @) between the mouth 
and the front margin of the carapace, 
thus occupying the same position as the 
hypostome of Trilobites (p. 233). The 
metastoma is always well developed and 
forms one of the distinguishing features 
of the Eurypterids ; in form it varies from 
oval in Lurypterus, to cordate in Slimonia, 
and lyrate in Dolichopterus. 
The principal modifications seen in 
the genital operculum are in the form 
of its median process; in S/imonia this 
either ends in three sharp points posteriorly 
(Fig. 165, ¢), or has the form of a trun- 
eated cone; its form in Hurypterus has 
already been described. — Glyptoscorpius 
differs from other Eurypterids in the 
possession of comb-like organs closely 
resembling the pectines of Scorpions. 
Slimonia apparently differs from other 
genera in that the plate-like appendages 
on the posterior three segments of the 
mesosoma do not meet in the middle line 
(Fig. 165, 10-12). Im some forms, such 
as Pterygotus (Fig. 164), there is a nearly 
gradual decrease in the width of the 
Fic. 165. — Slimonia acu- 
minata, Salter. Upper 
Silurian. Restoration of 
ventral surface, x 4. 1- 
6, Appendages of  pro- 
soma; 7, 8, genital oper- 
eculim ; 7-12, mesosoma ; 
13-18, segments of meta- 
soma ; 19, tail-spine.; a, 
lateral eye ; 6, metastoma, 
covering the inner parts 
of the coxae of the last 
pair of appendages ; c¢, 
median process of genital 
operculum ; d@, branchial 
lamellae seen through the 
plate-like appendages. 
(After Laurie. ) 
segments in passing 
