ARTHROPODA. EURYPTERIDA. 209 



shield carries on its upper surface two pairs of eyes; a 

 large compound pair (d) placed laterally or at the anterior 

 angles, and a pair of simple ocelli (e) placed close together 

 near the centre of the shield. The mouth is on the under 

 surface of the cephalothorax (fig. 56); in front of the 

 mouth there is one pair of appendages only (c), they are 

 jointed and often end in chelae, and are considered to 

 represent the antennae ; the other four or five pairs of 

 appendages (!' 5') consist of seven or eight joints each 

 and are sometimes chelate, they function both in mastica- 

 tion and swimming, the posterior pair (5') is often very 

 much larger than the others. Placed just behind the 

 mouth, in the median line, is an oval or heart-shaped plate 

 the metastoma (b). The six segments of the abdomen are 

 not continued round on the ventral surface as would at 

 first sight appear. But there are instead five plate-like 

 appendages which cover the gills (I. V.) ; the first, known 

 as the operculum, is often divided into two parts by a 

 median process, and corresponds with that of Limulus. The 

 segments of the post-abdomen (7 12) are continued on 

 the under surface and bear no appendages. 



The Eurypterids agree with the Xiphosura in many 

 points, but differ in the abdominal segments being always 

 free, in the possession of a metastoma, and in the absence 

 of trilobation. 



Eurypterus. Cephalothoracic shield quadrate; the 

 compound eyes are not at the margin, but a little in front 

 of the median lateral point on each side. The telson is 

 greatly elongated. The pre-oral appendages are small 

 and not chelate ; of the five other pairs of appendages the 

 posterior is very much the larger. The metastoma is 

 oval. Lower Ludlow to Carboniferous. 



w. P. 14 



