248 THE CHICAGO ACADEMY OF SCIENCES. 



studded with small, crowded, depressed, rounded tubercles. 

 The cephalon and thorax are unknown. 



The approximate dimensions of an average sized pygidium 

 are: length 58 mm., width 68 mm., width of axis at anterior mar- 

 gin 17 mm. 



Remarks. These pygidia are essentially like those which 

 occur in the fauna of the Waldron shale of Indiana, which have 

 always been considered as a variety of A . boltoni of the New York 

 Niagaran fauna. The characters of all these western pygidia 

 seem to be quite constant and to differ from the typical A. 

 boltoni in having the pleural segments more elongate and di- 

 rected more nearly in a posterior direction. Because of the 

 constancy of these characters the form is' here recognized as 

 having full specific rank. Wherever it occurs the species is 

 known almost wholly from the pygidium, and no heads cer- 

 tainly belonging to it have been observed in -the Chicago area. 

 However, one incomplete lichad glabella showing parts of a 

 broad, moderately convex median lobe and a single pair of 

 smaller lateral lobes, with the marginal border produced in 

 front in a flat lip-like extension, perhaps represents the cephalic 

 portion of this species. 



The species Lichas boltoni, with which M. occidentalis is 

 certainly co-generic, has been used at different times as the type 

 for three different genera or sub-genera of the Lichadidce. Cas- 

 telnau first used the name Arctinurus in 1843*, Conrad, however, 

 had used the name Platynotus in 1838!, but this name had 

 been preoccupied for a genus of insects, and so cannot be re- 

 tained. In 1885 Schmidt proposed the name OncholichasJ., 

 with L. boltoni as the type, associating with it certain Baltic 

 Ordovician species which probably are not co-generic. In the 

 present paper Castelnau's name is retained for the generic group. 



Locality. Joliet and Bonfield, Illinois. 



Arctinurus chicagoensis, n. sp., pi. xxm, figs. 7-8; pi. xxii, fig. 14. 



Description. The entire head not known. Glabella sub- 

 pentagonal in outline, strongly convex, bounded laterally by 

 strongly defined dorsal furrows; the median lobe extending 

 from the anterior margin of the head to the occipital furrow, 

 broadest in front where it occupies more than one-third the 



*Ess. Syst. Sil. 1'Amer. Sept., p. 21. 

 tRep. N. Y. State Geol. Surv. for 1838, p. 118. 



JRev. Ost, Bait. Sil. Tril., pt. 2, Mem. Acad. Imp. Sci. St. Petersburg. Vol. xxxiii. 

 No. 1, p. 3. 



