Raymond & Narraway : Notes on Ordovician Trilobites 243 



description can be drawn up which will include them all, but if they 

 are all put under one name, there is nothing to show the fact that 

 certain forms have been isolated and have developed into a race with 

 fixed characters. Some may question the propriety of a course which 

 seems to give to geographic variations the rank of species, and we our- 

 selves should have hesitated to give a new name under such conditions, 

 even though we feel that there should be some way of designating 

 such forms. In this particular instance however, it is only necessary 

 to revive a name previously given by an acute student of these 

 organisms. 



The following is a list of the trilobites which have thus far been 

 found in the Black River Limestone in the vicinity of Ottawa. 

 Bathyurus extans (Hall), Illcenus latiaxiatus sp. no v., 



B. spiniger (Hall), Thaleops ova fa Conrad, 



Asaphus romingeri Walcott, Bu?nastus milleri (Billings), 



Isotelus gigas Dekay, Bumastus indeterminatus (Walcott), 



Illcenus conradi Billings, Ceraurus pleurexanthemus Green, 



/. angusticollis Billings, Cybele ella Narraway and Raymond, 



Pterygometopus callicephalus (Hall). 



Sub-Kingdom ARTHROPODA. 



Class CRUSTACEA. 



Sub-Class TRILOBITA. 



Order OPISTHOPARIA Beecher. 



Genus Illaenus Dalman. 



Illaenus latiaxiatus sp. nov. 



(Plate LX, figures 4-8.) 

 This species presents many points of similarity to Illcenus ameri- 

 canus Billings, but the pygidia are so markedly different that we are 

 unable to refer them to that species. No complete specimens have 

 yet been found, but enough material is at hand to enable us to describe 

 all the parts except the free cheeks. 



Description. 

 Cephalon apparently a little less than half as long as wide, strongly 

 arched, nearly semicircular in outline. The dorsal furrows are deep 



