XIPHOSURA. 



385 



The Xiphosura seem to have commenced existence in the 

 Upper Silurian period, where they are represented by the 

 Neolimidus falcatus of Dr Henry Woodward. In this re- 

 markable genus (fig. 241) the head -shield has a general 

 resemblance to that of the recent Lvm/ulus, and there are 

 traces of a divisional line crossing the head and apparently 

 corresponding with the " facial suture " of the Trilobites. 

 Compound eyes and ocelli seem to be present, and there 

 are six free thoracic segments, followed, probably, by three 

 -free abdominal rings, of which only two have been preserved. 

 A long spiniform telson may be conjecturally added to com- 

 plete this ancient Limuloid Crustacean. In the Devonian 



Fig. 241. Keoliinuliis falcatus, enlarged 

 about three times. Upper Silurian. (After 

 Henry Woodward.) 



Fig. 242 Prestwichiu rotundata. 

 Coal-ineasures. 



no certain traces of Xiphosurans have been yet detected, but 

 several types occur in the Carboniferous. The most import- 

 ant of these is the genus Prestwichia (fig. 242), which has 

 the general form of Neolimulus, but in which the thoracic 

 and abdominal segments are not marked off from one an- 

 other, and are all anchylosed. The genus Euproops, from 

 the Coal-measures of North America, is hardly separable 

 from the preceding, but the eyes are situated on the anterior 

 edge of the cephalic buckler. Another well-known Carbon- 

 VOL. I. 2 B 



