R.WMoxi) : Notes on Ordovician Trii,oi!Itf,s. 



43 



specimens from the same horizon at Cohmibia, Xcw Jersey, and referred 

 the species to Illceniirus. In this genus Cleland's name was preoccupied, 

 and therefore Welier was forced to give a new name. 



The genus Illceniirus was proposed by Hall' for a trilobite found near 

 the middle of the Potsdam sandstone near Osceola Mills, Wisconsin. This 

 genus appears to be closely related to Symphysitrus, as Brogger has al- 

 ready suggested ("Euloma-Niobe Fauna," p. 72). There is, however, 

 a real difference between the two genera, for Symphysurus has a 

 narrow axial lobe in the thorax, and a pygidium nearly as long as wide, 

 with a distinct axial lobe. In Illceiiiiius the axial lobe of the thorax is 

 very wide, as in Xileiis, the pygidium is twice as wide as long, and has no 

 trace of an axial lobe. 



8 9 



Fig. 8. Symphysurus sicardi (Bergeron). Outline of a specimen in the Carnegie 

 Museum. 



Fig. 9. Illanurus quadralus Hall. Outline restored from the various parts 

 figured by Hall. The axial lobe of anterior thoracic segments has been made a 

 little too wide. 



The pygidium of the species described by Cleland and W'eller is nearly 

 as long as wide and has a distinct axial lobe, thus agreeing with Sym- 

 physurus rather than IllcBnurus. The thorax of this species is not known, 

 but it is highly probable that the axial lobe is narrow, as it is in all trilo- 

 bites whose pygidia have narrow axial lobes. It will be noticed that the 

 glabella in Ilemigyraspis and Asaphellus is broad, but the axial lobe on 

 the pygidium is narrow, as is the axial lobe of the thorax also. In Isotelus, 

 Nileus, and Illanurus quadratus, where the axial lobe of the thorax is 

 very broad, there is almost no trace of an axial lobe on the pygidium. 

 Symphysurus convexns is \-ery similar to Symphysurus angustatus Sars 

 ^i6lh Report New York Stale Cabinet Natural History, 1863. 176, pi, 7. 



