RAYMOND: NEW AND OLD SILURIAN TRILOBITES. 15 
cave border on either shield. Eyes very large. Type, Bumastus 
barriensis Murchison. Ordovician and Silurian. North America. 
Silurian. Europe. 
Actinolobus Eichwald. Axial lobe narrow, cephalon short, pygi- 
dium long, with very wide concave border. Eyes large. Type, 
Illaenus atavus Eichwald. Ordovician, Russia. Silurian. Russia 
and United States. 
Illaenoides Weller. Axial lobe with a width between that of Actino- 
lobus and Bumastus, eyes very small and far forward, narrow con- 
cave border on pygidium. Type, Jllaenoides trilobus Weller. Silurian. 
United States. 
The above classification is designed to separate the species with 
long more or less flattened shields from the more typical illaenids with 
short and abruptly deflected cephalon and pygidium. The first two 
genera are modifications of the central Illaenus type, the other three 
of the more flattened Dysplanus group. In defining the subfamily 
Bumastinae as I have, all the forms with a more or less Isotelus-like 
pygidium are removed from the typical Il]aenus group. Among the 
small species constituting the earliest of the Bumastinae one finds 
species like B. globosus Billings, B. bellevillensis Raymond and Narra- 
way, and a few others, which lack a concave border. On the other 
hand, so large a Middle Ordovician Bumastus as B. indeterminatus 
(Walcott), the type of which is figured (Plate 2) for the first time, has a 
distinctly concave border. It is very possibly true that the small 
species mentioned above should be given a distinct name and placed 
in the Illaeninae, but it still seems somewhat early to take so radical 
a step. 
Description of species. 
We owe to Professor Weller a complete and careful description of 
the Illaenidae of the Chicago area, and, as he had access to collections 
made at Racine and near Milwaukee, his description in large measure 
covers the Wisconsin area also. In the large collections which I have 
been able to examine, I have, however, found a few specimens more 
perfect than those previously described, and also a few new species. 
When first studying the excellent figures given by Weller, one is struck 
by the apparent triviality of the specific characteristics employed in 
the discrimination of the species, but with a large collection, it is 
found that the characteristics are remarkably constant. The study 
of these illaenids is unusually interesting, in fact, for it seems to be 
