256 



MINERALS AND GEOLOGY 



nine segments) and pygidium, are nearly of equal size. The 

 usually extends to within *a short distance of the extremity of the 

 pygidium, and the latter has very frequently a marginal furrow. 

 The genus Bronteus is mostly a Devonian and Upper Silurian type, 

 but is represented also in Lower Silurian strata. It is distinguished 

 by its ten body-segments with slightly-raised bands in place of fur- 

 rows on the pleurae, and its widely-expanded glabella ; and especially 

 by its large pygidium, with short axis, and fan-like, furrowed sur- 

 face, the furrows curving upwards. The genus Lichas is exclusively 

 Silurian. It is distinguished chiefly by its nine or ten body-segments ; 

 its broad, backward-curving, and pointed pleurae ; and its compara- 

 tively large pygidium with short axis, largely-serated margin and 

 almost longitudinal furrows. The genus Dikelocephalus is exclusi- 

 vely Cambrian. From this latter circumstance it is comm nly 

 placed in the family of the Olenidce, in despite of its very dissimilar 

 aspect and structural characters. Whilst Olenus (with other 

 members of the Olenidae family)has a very small pygidium, compared 

 with the broad head-shield, in Dikelocephalus the pygidium is as 

 large as the head-shield or even larger, and it lias a very short axis 

 as in Lichas and Bronteus of this group. The genus, however, is 

 still very imperfectly denned. The number of 

 the body-segments is not yet known ; and the 

 glabella, as regards its transverse furrows, 

 appears to differ very considerably in different 

 species. Figure 171 represents the pygidium 

 of D. magnificus (after Billings) from the Levis 

 or Quebec formation. In some other species 

 referred to this genus, the outline of the 

 dium is simply rounded. 



FIG. 171. 



Dikelocephalus magnifi- 



cua : Pygidium, after 



Billings. Levis 



Formation. 



The Frontones are distinguished by a very 

 large head-shield and small, or comparatively 

 small, pygidium. The glabella is more or less broad and prominently 

 developed, and the posterior angles of the head-shield are almost 

 always in the form of long spines or horns. The more typical 

 genera comprise : Paradoxides, with from sixteen to twenty spinous 

 body-segments and very small pygidium ; Trinucleus, with six body- 

 segments, large oval glabella, and head-shield surrounded by a per- 



