May, 1919] Silurian Fossils From Ohio 389 



to that of one or two of the posterior rings. The pleural lobes are 

 marked by three distinct ribs and a fourth, much less distinct rib, 

 leaving an unoccupied space at the posterior end of the axial lobe suffi- 

 cient for the insertion of a fifth rib, which, however, does not occur. 

 The pleural lobes are convex, excepting toward the margin, where, for a 

 space about as wide as the distance from the termination of the axial 

 lobe from the posterior margin of the pygidium, the curvature is dis- 

 tinctly concave along the entire margin. 



Proetus determinatus, Foerste. 

 Plate XIX, Figs. 14 A-D. 



18S7. Foerste. Bull. Sci. Lab. Denison Univ., vol. 2, p. 91, pi. 8, figs. 2, 3, 3a. 

 1913. Savage. Bull. Geol. Surv. Illinois, No. 23, p. 104. pi. 6, figs. 10, 11. 



Proetus determinatus as identified by Savage from the gray lime- 

 stone in the upper part of the Edgewood limestone, near Thebes, 

 Illinois, differs from Proetus princeps chiefly in the smaller size of the 

 occipital lobe occurring at each end of the neck ring, and in the presence 

 of distinct furrows along the segments of the pleural lobes of the 

 pygidium. 



The lobe at each end of the neck ring evidently originated from the 

 neck ring itself, since only a weak furrow separates it from the latter; 

 this furrow does not actually reach the posterior margin of the neck 

 ring, but reaches the dorsal furrow a slight distance anterior to this 

 posterior margin; it is most distinct where it leaves the neck-furrow. 

 A small granule ornaments the median part of the neck ring. The 

 glabellar furrows are faintly defined or obsolete. 



In both Proetus determinatus and Proetus princeps, the concave 

 flexure limiting the posterior part of the anterior border is separated 

 from the anterior margin of the glabella by only a short distance. 



It is probable that the faintly defined or obsolete glabellar furrows 

 are represented, on the under side of the cranidium, by much more 

 strongly defined ridges, and that freqtiently the presence of the latter 

 may be detected through the partially translucent test. 



Proetus princeps, Savage. 

 Plate XIX, Figs. 13, A, B. 

 1913. Bull. Geol. Surv. Illinois, no. 23, p. 57, pi. 2, fig. 14. 



The cranidium figured by Savage is characterized by the large size 

 of the lobe occurring at each end of the neck ring. These lobes appear 

 to have originated from the neck-ring, having been cut off from the 

 latter by oblique furrows. 



The axial lobe of the' pygidiinn is marked by about 10 rings, rather 

 faint near the posterior end of the lobe. The grooves along the ribs 

 of the pleural lobes are either very faint or entirely obsolete. Usually 

 only the three anterior pairs of ribs show traces of these grooves, if 

 any are present. 



Locality and Position: Girardeau limestone, near Thebes, 

 Illinois. 



