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12 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
should probably conclude at once that the Goldiidae were derived 
from the IIlaenidae. 
The oldest illaenids which are well known are those which Barrande 
described from the base of the Ordovician in Bohemia. Of these, 
Illaenus advena is a quite typical Tlaenus, while the other species are 
peculiar. Illaenus bohemicus, the type of which is in the M. C. Z., 
was founded on a badly preserved single specimen, which does not 
seem to be an Illaenus. The pygidium shows a long, distinct, and 
ringed axial lobe and if this member were found alone, it would at 
once be assigned to the Asaphidae. The nine segments of the thorax 
are, however, without pleural furrows. The cephalon is too poorly 
preserved to indicate any characters of value. Long wide genal 
spines are present, the glabella appears to have nearly parallel sides 
until the vicinity of the anterior end is reached, when it expands 
abruptly. Rather faint glabellar furrows seem to be present. With 
only this single specimen, it does not seem possible to assign the 
species to any genus or even family, and it certainly throws no light 
on the origin of the Ilaenidae. Another species from Dy, described 
by Barrande, is the common Illaenus katzeri. This species differs 
from other illaenids in its eyes. Barrande supposed it to be blind, 
but Holub has recently shown (Bull. international Acad. sci. Bohéme, 
1908. German abstract, p. 7, pl. 7) that it has small eyes, situated 
forward. Both shields are rather flattened, not so strongly incurved 
as in the typical genus Illaenus, and the pygidium is long and parabolic 
in outline, thus suggesting Dysplanus. There is nothing, however, 
to indicate that J. katzeri is primitive, but it seems rather, like the 
Silurian [/aenoides trilobus, to be a degenerate form. If the genus is 
divided at all, this species cannot be considered as congeneric with 
Illaenus crassicauda, and I would suggest that the name Wossekia 
be applied to it, since Wossek, Bohemia, is the locality from which 
practically all the specimens have been obtained. Jllaenus puer 
seems to be, as Brégger was the first to point out, a Svmphysurus. 
Illaenus calvuus has a peculiar marginal rim. The species is known 
from a single fragment. Jllaenus aratus has a narrow glabella, and 
the eyes are very far apart or absent. It is primitive for an illaenid, 
but gives no suggestion as to the origin of the group. 
A pathologic specimen of Illaenus.— Since this paper was written, 
I have been enabled, through a grant from the Shaler Memorial fund 
at Harvard College, to visit northern Europe, and some time was 
spent in studying the collections in London, Berlin, St. Petersburg, 
Stockholm, and Christiania. One of the most interesting trilobites 
seen was the specimen of [//aenus revelensis Holm, which was figured 
