THE TRILOBITA AND OTHER ARTHROPODA 197 



be moulted. The portion of each cheek lying external 

 to the suture is called the free cheek, the remainder is the 

 fixed cheek, and the glabella together with its attached 

 fixed cheeks constitutes the cranidinm. Loose cranidia 

 and free cheeks are often found, probably detached in 

 the process of moulting. In all trilobites which possess 

 both eyes and facial suture, the eye lies on the free 

 cheek in contact with the suture, and the adjacent part 

 of the fixed cheek has a protuberance fitting into the 

 outline of the eye, called the palpebral lobe. 



If the suture cuts the lateral margin in front of the 

 genal angle, it is described as proparian ; if shifted so 

 as to cut the posterior margin, it would be opistho- 

 parian. That of Calymene is on the verge between the 

 two. 



In specimens which show the under-side of the head, 

 there is seen, articulated to the central part of the 

 marginal rim, a somewhat oblong plate with indented 

 posterior margin. This is called the labrum (or hypostome), 

 and formed a biting upper-lip to the mouth (Fig. 61). 



The pygidium of Calymene is nearly semi-circular, the 

 free posterior margin having much less curvature than 

 the anterior margin which articulates with the thirteenth 

 thoracic somite. The axis tapers backwards to a rounded 

 termination, a little short of the actual end of the pygi- 

 dium : its anterior two-thirds are divided into six well- - 

 marked somites, the remainder is unsegmented. There 

 are six pairs of pleura, which from front to rear show an 

 increasing backward curvature, until the last pair run 

 straight back parallel to one another. The margin is 



