RAYMOND: NEW AND OLD SILURIAN TRILOBITES. 13 



Ly Holm in his description of the Russian Ordovician Illaenidae 

 (Mem. Acad. imp. sci. St. Petersburg, 1886, ser. 7, 33, pi. 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 nvunber 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. 



