376 



ARTHEOPODA. 



marked. There are eight to ten body-rings ; and the axis 

 of the tail is truncated or wanting, and in no case exhibits 

 definite segmentation. The free cheeks are extremely small ; 

 the facial sutures are separate ; and the eyes are crescentic. 



If Nileus be regarded as belonging to the Asaphidce, the 

 only well-defined genus in this family is Illcenus itself (fig. 

 234). In this genus the head is large, convex, and tumid, 

 as is also the tail, the glabella and pygidial axis being hardly 

 marked out, and being wholly unsegmented. In the typical 

 forms of this group (Illcenus proper), the axis of the thorax 

 is not disproportionately wide, and the axal furrows are dis- 

 tinct ; but in others (Bumastus), the thoracic axis is extremely 

 wide, and is hardly separated from the pleurae by recognisable 

 axal furrows. 



Fig. 235. A, JEglina prisca Upper Cambrian; B, Head-shield of Cheirunts juvenis Lower 

 Silurian ; c, Head-shield of Amphion Fischeri Silurian ; D, Side view of the head-shield of 

 Sphwrexochus mirus Lower Silurian. (After Barrande and Salter.) 



1 4. ^EGLINID M. This family contains only the single 

 genus jffiglina, (fig. 235, A), of the Upper Cambrian and 

 Lower Silurian, and is chiefly distinguished from the preced- 

 ing by the much larger size of the eyes, and the smaller 

 number of body-rings. The head and tail are both of large 

 size, the latter with a truncated axis ; the glabella is not con- 

 spicuously marked out ; the facial sutures are discontinuous ; 



