206 ARTHROPODA. XIPHOSURA. 



covered dorsally by a large horse- shoe shaped shield, 

 which carries on its upper surface two pairs of eyes, one 

 compound, near the middle of the lateral parts, the other 

 simple and placed close together near the anterior margin. 

 Behind the cephalothorax, and movably articulated with 

 it, comes the abdominal shield ; this is composed of six 

 fused segments, the segmentation being shown by grooves ; 

 at the sides it bears movable spines, and posteriorly it is 

 provided with a long caudal spine. The cephalothorax 

 bears six pairs of appendages on its under surface, the 

 anterior pair only being placed in front of the mouth ; 

 the basal joints of the five posterior pairs function in 

 mastication. The abdomen carries six pairs of appendages, 

 the anterior pair are plate-like and serve as an operculum 

 to cover the remaining five pairs, these bear the gills 

 which consist of a large number of thin plates. 



The earliest Xiphosurans (Hemiaspis, Neolimulus) are 

 found in the Silurian ; in the Carboniferous, Prestwichia, 

 Belinurus and others occur. Limulus appears first in the 

 Trias. The Palaeozoic genera differ in some respects from 

 the later ones; the head-shield is not fused with the 

 thoracic shield, and the former sometimes possesses a 

 facial suture ; generally also, some or all of the segments 

 of the thorax and abdomen remain free and movable. 

 Thus in Hemiaspis there are six segments in the thorax 

 and three in the abdomen, all of which are free: in 

 Belinurus there are five free thoracic segments and three 

 fused abdominal ones ; in Prestwichia all the segments of 

 both thorax and abdomen are fused. 



ORDER. EURYPTERIDA. 



The Eurypterids are quite extinct and form a very 

 remarkable group of the Arthropoda ; they are usually of 



