RAYMOND: NEW AND OLD SILURIAN TRILOBITES. 13 
by Holm in his description of the Russian Ordovician Illaenidae 
(Mem. Acad. imp. sci. St. Petersburg, 1886, ser. 7, 33, pl. 2, f. 5a). 
This is the only example I have seen among trilobites of a malforma- 
tion due to moulting. As partially shown in Holm’s figure, there is 
an impressed line between the facial suture and the dorsal furrow on 
the left side, which follows exactly the course of a facial suture, even 
extending across the front of the cranidium, a point not shown in 
Holm’s figure. Across the posterior part of the cephalon, close to the 
edge, is a furrow marking the posterior edge of the shell at the time 
of the previous moult. The eye on the left side is smaller than its 
opposite, and the palpebral lobe is malformed. The cephalon is 
decidedly unsymmetrical, the left free cheek being drawn backward. . 
All these pathological features seem to be due to the partial retention 
of the shell at the next previous moult. 
Classification of the Illaenidae. 
From the first, all classifications of the Illaenidae have rested mainly 
upon the number of segments in the thorax, and secondarily upon the 
width of the axial lobe of the thorax. Thus, Dalman, the describer 
of Illaenus, separated the species with nine segments from those with 
ten. Holm, the principal writer upon the genus, while recognizing 
only the genus Illaenus and the subgenus Bumastus, divided Illaenus 
into three groups, those with ten, nine, and eight segments in the 
thorax. A study of the American illaenids does not favor a classifica- 
tion of this sort, for it has been repeatedly shown that the number of 
thoracic segments in species of both Illaenus and Bumastus is variable, 
even within the limits of a single species. Likewise, the presence or 
absence of genal spines’is not a characteristic justifying, in itself, the 
erection of a genus of Illaenidae, for, as has been several times pointed 
out, species are found with all sorts and conditions of spines, and if all 
species having this characteristic were to be referred to a single genus, 
- there would be hardly another characteristic common to the assem- 
blage. The length and convexity of the cephalon and pygidium, the 
size and position of the eyes, the width of the axial lobe of the thorax, 
and the shape of the glabella seem of the most importance, but I would 
also take into consideration the sort of genal spines which may be 
present. It is still too early to make any natural classification, and 
the genera here recognized are based primarily upon the more con- 
spicuous peculiarities of the type-species of each. 
