10 bulletin: museum of comparative zoology. 



scribed by Hicks as /. hughesi, and not understanding clearly the 

 characteristics of either species, produced a name of no particular 

 value. The name should be dropped, unless it can be shown to be of 

 more value than now appears to be probable. 



Hydrolaenus Salter, 1867. 



Type, Illaenus conifrons Billings. 

 Monog. Brit. Silurian trilobites, 1867, pt. 4, p. 182. 



The type-species is a Thaleops, and Hydrolaenus is therefore a 

 synonym of that genus. 



Illaenoides Weller, 1907. 



Type, Illaenoides trilobus Weller. 



Bull. Chicago acad. sci., 1907, no. 4, pt. 2, p. 226. 



The type of this genus is remarkable chiefly for its small eyes which 

 are situated halfway to the front of the head. The glabellar furrows 

 are narrow and shallow, the facial suture cuts the cheeks very close 

 to the genal angles, making the free cheeks of unusual shape. The 

 genal angles are rounded. The axial lobe of the thorax is wider than 

 in typical Illaenus, but less wide than in Bumastus. The pygidium 

 is long with a narrow concave border, but no trace of an axial lobe. 

 Type and only known species from the Niagaran at Bridgeport (Chi- 

 cago), Illinois. 



Su7nmary. — It appears that of the genera proposed, Illaenus, 

 Bumastus, Thaleops, Actinolobus, and Illaenoides have unquestion- 

 able value. Dysplanus and Octillaenus are more or less valuable but 

 need further study and redefinition. Cryptonymus, Deucalion, 

 Panderia, and Ectillaenixs are names which cannot be used at present, 

 but might possibly be revived. Archegonus, Illaenurus, and Illae- 

 nopsis belong to other families. Alceste was never properly defined. 

 Rhodope was preoccupied, and Hydrolaenus is a synonym. It is in- 

 teresting to note the lapse of forty years between Salter's new names 

 and the next generic name applied to a member of this group. 



It is evident that the type-genus Illaenus contains the great majority 



