10 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
scribed by Hicks as I. 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. 
HypDROLAENUS 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. 
Summary.— It appears that of the genera proposed, Illaenus, 
Bumastus, Thaleops, Actinolobus, and IIlaenoides have unquestion- 
able value. Dysplanus and Octillaenus are more or less valuable but 
need further study and redefinition. Cryptonymus, Deucalion, 
Panderia, and Ectillaenus are names which cannot be used at. present, 
but might possibly be revived. Archegonus, Ilaenurus, 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 thatthe type-genus Illaenus contains the great majority 
