208 BRITISH SPECIES OP FALSE-SCORPIONS. 



DESCRIPTIONS OF THE GENERA AND SPECIES. 



Group I. Eyes four. 



Two British Genera only are comprised in this group 

 Chthonius C. Koch and Obisium Leach. 

 These may be thus distinguished : 



i. Claws of pincers, straight, Chthonius. 

 ii. Claws of pincers, curved, Obisium. 



GEN : CHTHONIUS (C. Koch.) 



The Cephalothorax is equal in length and breadth ; it is either 

 squarely or a little roundly truncated in front, and the eyes are 

 placed two on each side of the anterior part of the caput, more or 

 less separated from each other in a line parallel with the plane of 

 the Cephalothorax. The abdominal segments are eleven ; those on 

 the upper side entire (i.e., not divided by a longitudinal central 

 divisional line). Fakes, strong, nearly as long as the Cephalothorax. 

 Palpi, rather long ; the radial and digital joints (forming the fore- 

 cept or pincers) are very long, and the fingers straight. The femora 

 of the third and fourth pairs of legs are very strong and compressed, 

 and the tarsi are divided. The first and second pairs devoid of a 

 " trochantin ;" surface of the integuments, smooth and shiny. 



Four species only are as yet known in Britain C. orthodactylus 

 Leach, G. Rayi L Koch, C. tetrachelatus Preyss, and 0. tennis 

 L. Koch. 



These may be shortly distinguished as follows : 



1. Cephalothorax of equal width throughout, C. tetrachelatus. 



2. Cephalothorax wider in front than behind. 



(a) Bulbous portion of pincers unicolorous, or nearly so, with 

 the rest of the palpus. 



i. Size smaller ; eyes separated by a less interval ; 

 Cephalothorax less wide in front. C. orthodac- 

 tylus. 



ii. Size larger ; eyes separated by a greater interval ; 

 Cephalothorax more widened in front. C. Rayi. 



(b) Bulbous portion of pincers black-brown, C. tennis. 



