294 ARACHNIDA—EURYPTERIDA CHAP. XI 
of legs are toothed and meet in the middle line, but in the 
Scorpions the coxae of the last two pairs do not meet; this 
difference, however, appears to be bridged over in the earliest 
known Scorpion—Palacophonus, from the Silurian rocks. The 
Eurypterids are distinguished from the Scorpions by the much 
greater development of the last pair of legs. The large meta- 
stoma of the former is homologous with the sternum of the 
Scorpion. The genital operculum is much smaller in Scorpions 
than in Eurypterids, and in this respect the latter agree with 
Thelyphonus (one of the Pedipalpi) more than with the Scorpions. 
The pectines are absent in the EKurypterids except in Glypto- 
scorpius. Instead of the lung-books of the Scorpions the 
Eurypterids possess branchial lamellae on the plate-like append- 
ages; but this difference between the two groups appears to be 
bridged over by Palaeophonus, which was marine, and may have 
possessed branchial lamellae since stigmata seem to be absent. 
Gyptoscorpius, which is found in the Lower Carboniferous of 
the south of Scotland, is a form of considerable interest. It 
is about a foot in length, and agrees in many respects with 
Eurypterida, but it may be necessary to separate it from that 
group since it possesses pectines, and the legs end in a double 
claw ; it cannot, however, be regarded as a lnk between Euryp- 
terids and Scorpions, but must rather be considered as an offshoot 
from the former, since the latter group was already in existence 
at a much earlier period. : 
' Peach, Nature, xxxi., 1885, p. 295; Pocock, Quart. Journ. Mier. Set. xliv., 
1901, p. 291; Laurie, Trans. Roy. Soc. Edinb. xxxix., 1899, p. 575. 
2 Peach, Trans. Roy. Soc. Edinb. xxx., 1882, p. 516. 
