In I. Americanus- the posterior margin of the cheek outside of the eye 

 forms with the lateral margin, in some specimens, an angle of 90°, and in 

 others from 90° to 100°. In I. comimilis the same two lines (without 

 regarding in either case the rounded angle) are inclined towards each 

 other at an angle of less than 50° ; the posterior termination of the cheek 

 being produced into an auriculate projection, altogether different from 

 that of/. Americanus. 



I. Americanus is a rare species, but I have examined three si^ecimens 

 with the head, thorax and pygidium in connection, two others with the 

 head and thorax, and about thirty detached heads and portions of heads. 

 The comparison of these shews very conclusively that it is quite distinct 

 from /. consimilis. 



Locality and Formation. — ilost of the specimens are from Ottawa, 

 but we have it also from L'<_»rignal and Lake Huron ; Trenton limestone 

 only. 



iLLiENUS ISCEETUS. (N. Sp.) 

 a b ah c 



Fig. 319. 



Fig. 320. 



Fig. 319. — Illanus incerlus. a^ upper side of a perfect head ; 6, side view of the same. 

 Fig. 320. — /. consobrinus. a. upper side of the head ; 6, side view of the same ; c, side 

 view of another specimeD, with the posterior angles more produced.* 



Description. — Closely allied to I. consobrinus. Head strongly con- 

 vex, width twice the length, broadly rounded in front. Glabella wide, 

 depressed convex, its width equal to one-half the whole length of the head 

 following the curve ; dorsal furrows straight and parallel. Eyes of mode- 

 rate size, ovate, their length about one-third the width of the glabella, 

 about their own length from the posterior margin, and the same distance 

 from the dorsal furrow. The cheeks are subtriangular, narrowly rounded 

 at the angle, the lower margin a little concave between the angle and the 

 facial suture ; the posterior margin outside of the eye, in length, equal to 

 a little more than half the width of the glabella ; the lo\yer margin some- 

 what longer than the posterior. 



The pygidium closely resembles that of /. consimilis (^ante, p. 276), 



' In the figures of this species {ante, p. 280), the facial suture is represented as curv- 

 ing outvrards from the eye. On closely examining the specimens, I cannot malie out 

 the suture distinctly ; the line (only seen on one side) aiipearing to be a fissure. The 

 figures are therefore reproduced here with the line removed, to guard against error. 



