TRILOBITA—OPISTHOPARIA. 297 



103. I. imperator Hall. Siluric. 

 Cephalon without the free cheeks wider than long, strongly con- 

 vex. Eyes short and small, situated far back, almost at the pos- 

 terior border. Dorsal furrows strong, less than one half the length 

 of the head. Pygidium subelliptical, twice as wide as long, tri- 

 lobed anteriorly, the axial lobe occupying about one third the width. 



Niagaran of Illinois and Wisconsin. 



104. I. armatus Hall. (Fig. 1609.) Siluric. 



Fig. 1609. Illanus armatus, two views'of perfect cephalon, pygidium with last 

 thoracic segment. (After Hall, 20th Mus. Rep.) 



Rounded furrow present beneath the prominent eyes upon the 

 inner margin of the free cheeks. Free cheeks convex, ending in 

 rather short genal spines. 



Niagaran of Indiana, Illinois and Wisconsin. 



XLIII. BuMASTUS Murchison. 



Differs from Illcenus in practically lacking any longitudinal loba- 

 tion of the body except slightly on the cephalon. Thorax with 

 eight to ten segments. Ordovicic-Siluric. 



105. B. indeterminatus (Walcott). Ordovicic. 

 Dorsal furrows curved. Whole margin of cephalon marked by 



four or five furrows. Genal spines present. 



Chazy-Trenton of New York and Wisconsin; Black River of 

 New York, Ontario and Quebec. 



106. B. milleri (Billings). Ordovicic. 

 Differs from B. trentonensis in its broader and shorter form, 



more prominent dorsal furrows on cephalon, and thoracic segments 

 wider and always nine in number. 

 Black River of Ontario. 



