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 Palaeoplionusl 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 Eurypterids 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 PalaeopJionus, which was marine, and may have 

 possessed branchial lamellae since stigmata seem to be absent. 



Glyptoscorpius? 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 link 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. 



1 Peach, Nature, xxxi., 1885, p. 295 ; Pocock, Quart, Journ. Micr. Sci. xliv., 

 1901, p. 291 ; Laurie, Trans. Hoy. Soc. Edinb. xxxix., 1899, p. 575. 



2 Peach, Trans. Roy. Soc. Edinb. xxx., 1882, p. 516. 



