12 bulletin: museum of comparative zoology. 



should probably conclude at once that the Goldiidae were derived 

 from the Illaenidae. 



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 Illaenus, while the other species are 

 peculiar. Illaenus bohemicus, the type of which is in the M. (". Z., 

 was founded on a badlj^ preserved single specimen, which does not 

 seem to be an Illaenus. The pygidium sKows 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 

 imtil 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 Illaenidae. Another species from Di, 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. Boheme, 

 1908. German abstract, p. 7, pi. 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 /. katzeri is primitive, but it seems rather, like the 

 Silurian Illaenoides 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. Illaenus ])uer 

 seems to be, as Brogger was the first to point out, a Symphysurus. 

 Illaenus calvus has a peculiar marginal rim. The species is known 

 from a single fragment. Illaenus 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 imthologic 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 Illaenus revelensis Holm, which was figured 



