XIPHOSURA. 787 



cuticle bearing the appearance of lacquer, and parts of the 

 under surface are clothed with hairs. 



The cephalothorax is semicircular in outline, \\itti projecting 

 postero-lateral angles. It is convex above and presents three longi- 

 tudinal spinose ridges, a median and two lateral. A pair of median 

 eyes are situated one on either side of the anterior spine of the 

 median ridge, and two large lateral eyes, considerably further back 

 (at the level of the mouth) and external to the lateral ridges. On 

 either side of the median ridge is a longitudinal groove, both of 

 which, like the ridge between them, are continued back on the 

 abdomen and give rise to a tripartite division of the body recalling 

 that of Trilobites, a resemblance which the fuller knowledge 

 of the structure of these animals which we now possess shows 

 to be only superficial. The mouth is situated far back and 

 the under surface of the cephalothorax is deeply excavated to 

 allow play to the limbs which are grouped about it. 



The abdomen is movably articulated with the cephalo- 

 thorax, and deeply notched behind for the caudal spine. Its 

 lateral margins, which converge posteriorly, bear 6 movable 

 spines inserted into notches. Corresponding to these spines, 

 and to the limbs borne on the ventral surface, 6 paired depressions 

 are found in the longitudinal grooves on the dorsal surface, 

 indications of a segmentation of the anterior part of the abdomen 

 which is much more obvious in the embryo. Like the cephalo- 

 thorax the abdomen is deeply excavated below for the reception 

 of appendages. The caudal spine is movably articulated with 

 the abdomen and the anus opens ventrally at its base. 



A comparison with the Eurypterids and with Scorpio shows that the 

 region of the abdomen behind the last (6th) abdominal limbs represents 

 the six posterior segments of the bodies of these animals. The three 

 anterior (7th-9th) of these are for a brief period defined in Lhtnilus, by 

 the segmentation of the larva. 



A prominent longitudinal ridge, laterally flattened, the camero- 

 stome, lies in front of the mouth, in a portion corresponding with 

 the upper lip of Crustacea and the labrum of insects. Behind 

 the mouth is an oval cushion-like area, covered with short 

 hairs, the promesosternite of Lankester. The appendages of the 

 cephalothorax are grouped in a ring about the mouth and 

 this area. They are seven in number. The most anterior, 

 the chelicerae (I), are slender and chelate, consisting of three 



