ARACHNIDA. XIPHOSURA 341 



covers the remaining five pairs of appendages, which are 

 not united in the middle, and bear on their posterior 

 faces the leaf-like gills, of which there may be from 150 

 to 200 on each appendage superposed like the leaves in a 

 book. 



From the account given above it will be seen that 

 Limulus resembles the scorpions in several respects. In 

 both, the prosoma consists of at least six fused segments, 

 covered dorsally by a carapace which bears a pair of 

 median eyes and a pair of compound eyes. The meso- 

 soma of Limulus differs from that of the scorpions in 

 having the segments fused, and the metasoma of the 

 former is much reduced ; but in both there is a tail-spine 

 behind the anus. The prosoma bears six pairs of append- 

 ages which, in both cases, are similar in form and 

 position. On the mesosoma the genital operculum forms 

 the first pair of appendages; the second pair are the 

 pectines of the scorpions, and the first pair of plates which 

 bear gills in Limulus. The next four segments carry 

 lung-books in the scorpions and gill-books in Limulus. 

 From these characters, and from the absence of antennae, 

 it is concluded that Limulus is allied to the Scorpionida 

 rather than to the Crustacea as was formerly supposed. 

 The differences between the mesosoma and metasoma of 

 Limulus and the scorpions are, to some extent, bridged 

 over by some of the Palaeozoic Xiphosura described below. 



Limulus appears first in the Trias ; it has been found 

 in the Middle Jurassic of Northampton, and is common in 

 the Upper Jurassic of Solenhofen in Bavaria, and is also 

 represented in the Upper Cretaceous and the Oligocene. 

 In the Palaeozoic deposits — from Silurian to Permian — 

 several other Xiphosura occur ; most of these differ from 

 Limulus in having some or all of the abdominal segments 



