14 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
ILLAENIDAE Hawle and Corda. 
Opisthoparia with large, convex, nearly smooth, cephalic and 
abdominal shields. Epistoma large, hypostoma convex, ovoid. 
Thorax of eight to ten segments, with unfurrowed pleura. Pygidium 
without ribs, axial lobe short or absent. Ordovician and Silurian. 
ILLAENINAE, subf. nov. 
Illaenidae with narrow axial lobe, cephalon and pygidium without 
concave border. 
Illaenus Dalman. Cephalon and pygidium very convex, wider 
than long, abruptly deflected; eyes large and far back, axial lobe of 
pygidium high. Genal spines, when present, rounded in section. 
Type, Illaenus crassicauda (Wahlenburg). Ordovician and Silurian. 
Europe, North and South America, India, and Australia. 
Thaleops Conrad. Cephalon and pygidium similar to Illaenus, but 
with eyes on long stalks and elongate genal spines always present. 
Type, Thaleops ovata Conrad. Ordovician and possibly Silurian. 
North America. 
Dysplanus Burmeister. Cephalon and pygidium long and flattened, 
parabolic in outline, genal spines present, usually flattened. Type, 
Illaenus centrotus Dalman. Ordovician. Northern Europe. 
Wossekia, gen. nov. Cephalon and pygidium as in Dysplanus, 
genal spines absent, eyes small and far forward. Type, Illaenus 
katzert Barrande. Basal Ordovician. Bohemia. 
Octillaenus Salter. Similar to Dysplanus but with rounded genal 
spines, and pleura of first segment elongated into spines. Type, 
Illaenus hisingeri Barrande. Ordovician. Bohemia. 
BUMASTINAE, subf. nov. 
Illaenidae with (usually) concave border on one or both shields, 
axial lobe generally wide, though sometimes narrow. 
Bumastus Murchison. Axial lobe of thorax equal to more than 
half the total width; eight to ten segments. Typically, a concave 
border is present on the pygidium, frequently on both cephalon and 
pygidium, rarely (in small Middle Ordovician species) without con- 
