TRILOBITA. 



535 



genus Stygina is related to Ogygia, but the head-shield and pygidium 

 are very similar. Lastly, the Upper Cambrian genus Psilocephalus 

 forms a link between Asaphus and Illcenus ; while the genus Nileus 

 might be almost indifferently placed with either the Asaphtdce or the 

 Illcenidce. 



Family 8. Ill^enid^. — In this family the head and tail are greatly 

 developed, and, as in the Asaphidce, are approximately equal in size 

 (fig. 394). The glabella is broad and rounded, destitute of lobes, 

 and with the axal furrows hardly at all marked. There are eight to 



Fig. 2g2.— A sapkus (Isotehcs) platycephalus. 

 Ordovician. 



Fig. 393.— Asaphus (Megalas- 

 pis) exteiiuatus, of the natural 

 size. Ordovician, Sweden. (After 

 Angelin.) 



ten body-rings ; and the axis of the tail is truncated or wanting, and 

 in no case exhibits definite segmentation. The free cheeks are ex- 

 tremely 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. 394). 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 



