OF CENTRAL CANADA PART IV. 255 



side cheeks are often broken off, and the pygtdium is frequently 



FIG. 168. 



Asaphus platycephalus 

 ( = Isotelus gigas) : 

 Stokes. Trenton 

 Formation. 



H . = The hypostoma. 



FIG. 169. 



Asaphus Canadensis : Chap- 

 man. Utica Formation. 



found without the other portions of the body. The genus appears 

 to be exclusively Lower Silurian. Our most common species com- 

 prise ; A. platycephalus., a more or less smooth species with rounded 

 head angles and pleura?, very abundant in the Trenton limestone, 

 but occurring also in the Chazy and Hudson River Formations, 

 and A. Canadensis, with horned head-shield, pointed pleurae, 

 and furrowed pygidium, of common occurrence in the Utica bitumi- 

 nous schists of Collingwood, Whitby, Ottawa and other localities. 

 Closely related to Asaphus is the genus Ogygia, distinguished by its 

 shield-shaped or pointed hypostoma, and its laterally-furrowed pygi- 

 dium, but it is essentially a European type. The genus Illcenus is 

 represented by smooth, oval species, with large head-shield and pygi- 

 dium, feebly-raised glabella, far-apart eyes, ten (or rarely nine) body- 

 segments, and rounded pleurae. It is essentially a Lower Silurian 

 type but ranges from Upper Cambrian into Upper Silurian strata. 

 Figure 170 represents a species in a rolled up 

 condition from the Chazy Formation. / Ameri- 

 camis (= I. crassicauda ?) from the Trenton 

 limestone of the Ottawa district, is another 

 Canadian species; and fragmentary examples 

 of additional species occur in the Levis forma- 

 tion of Quebec. Bathyurus is also chiefly from 

 : t j ie game f orma ti O n. Its buckler, thorax (with 



