290 ARACHNIDA—-EURYPTERIDA CHAP. 
All the genera, of which about thirteen have been recognised, 
are placed in one family. 
Fam. Eurypteridae.—The carapace varies somewhat in out- 
line; in Slimonia it is more distinctly quadrate than in 
Fic. 164.— Pterygotus osiliensis, Schmidt, Upper 
Silurian, Rootzikiill, Oesel. Ventral surface. 
Reduced. (After Schmidt.) 1-6, Appendages of 
the prosoma; 7-12, mesosoma; 7, 8, genital 
operculum ; 13-18, metasoma; 19, tail-plate ; 
a, epistome ; 6, metastoma; c, coxae of sixth 
pair of appendages, 
Hurypterus, whilst in 
Pterygotus (Fig. 164) 
and Hughmilleria’ it is 
semi-ovoid. The lateral 
eyes are at the margin 
of the carapace in Ptery- 
gotus, Slimonia (Fig. 165, 
a), and Hughmilleria, but 
in the other genera, in- 
cluding the earliest form, 
Strabops, they are on the 
dorsal surface at a greater 
or less distance from the 
marein. 
The pre-oral append- 
ages of Pterygotus (Fig. 
164, 1) differ from those 
of other genera in their 
much greater length and 
in the large size of the 
chelae; they probably 
consist of a proximal 
joint and chelae only, 
although, commonly, they 
are represented as having 
a larger number of joints. 
Unhke Furypterus and 
Pterygotus, the second 
pair of appendages in 
Slimonia (Fig. 165, 2) 
differ from the third, fourth, and fifth pairs in being distinctly 
smaller and more slender, and it is 
probable that they were 
tactile. Whilst in Hurypterus the fifth pair of appendages are 
larger than the three preceding pairs, and also differ from them in 
1 Sarle, New York State Museum, Bulletin 69, 
Palaeont. 9, 1903, p. 1087. 
2 Beecher, Geol. Mag. 1901, p. 561. 
