TKILOBITA. 375 



rounded, and its margin always entire. This important 

 genus, though not unknown in the Upper Cambrian, is char- 

 acteristic of the Lower Silurian, in which it is represented 

 by numerous species, some of which attain the extraordinary 

 size of two feet in length. The genus Ogygia (also Lower 

 Silurian) is closely allied to Asaphus in general form and 

 proportions. The axis of the tail is, however, more conspic- 

 uously marked than in most Asaphi (fig. 233), the hypo- 

 stome is rounded and not cleft, the glabella is distinctly 

 furrowed, and the pleurse of the thorax have only rudimen- 

 tary "fulcra." Niobe, of the Upper Cambrian, is an early 

 form of this family, which is intermediate between Asaphus 

 and Ogygia, having the round hypostome and lobed glabella 

 of the latter, while it approaches the former in its wide 

 glabella and its obtuse and faceted pleurae. Barrandia and 

 Stygina are both related to Ogygia; and the Upper Cam- 



Fig. 234. A, A complete example of Illcenus Davisii, in its unrolled state ; B, Hypostome of 

 the same ; c, Illcenus (Bumastus) Barriensis, rolled up. Lower Silurian. (After Salter.) 



brian genus Psilocephalus forms a link between Asaphus and 

 lllwnus ; while the genus Nileus may be almost indifferently 

 placed with either the Asaphidce or the Illcenidce. 



13. ILL^ENID^E. In this family the head and tail are 

 greatly developed, and, as in Asaphidce, are approximately 

 equal in size (fig. 234). The glabella is broad and rounded, 

 destitute of lobes, and with the axal furrows hardly at all 



