EXPLANATION OF PLATE XVIII. 



Figures 1-3 ILLAENUS icxus Hall p. 222 



Dorsal, anterior and lateral views of a complete cephalon, from 

 Joliet. (Pal. Coll. Walker Museum, No. 10, 729, Van Home Col- 

 lection). 



Figures 4-6 ILLAENUS ARMATUS Hall p. 222 



Lateral, anterior and dorsal views of a complete cephalon, from 

 Lockport. (Pal. Coll. Walker Museum, No. 9,896, Van Home 

 Collection). 



