246 THE CHICAGO ACADEMY OF SCIENCES. 



backward toward their inner extremities; posteriorly the axis 

 is produced into a depressed post-axial region, the dorsal fur- 

 rows converge posteriorly to about the middle of the pygidium 

 where the axial region is about one-half its width at the ante- 

 rior margin, from here the dorsal furrows diverge posteriorly, 

 the axial region again becoming broader, the furrows do not 

 reach the posterior margin of the pygidium so that the axial 

 region is coalescent posteriorly with the pleural segments on 

 either side. The pleura are divided into three pairs of broad 

 grooved segments, the posterior-lateral angles of the two an- 

 terior pairs project slightly beyond the border of the pygidium 

 as posteriorly pointing free points, between the free points of 

 the second pleural segments the border is continuous. 



The entire surface of the head, pygidium and thoracic 

 segments is covered with small, more or less irregular, rounded 

 papillae. 



The dimensions of the best preserved cranidium observed 

 are: length 17.5 mm., approximate width between the eyes 

 18.5 mm., width at posterior margin 30 mm., width of median 

 lobe of glabella in front 13 mm. The best preserved pygidium 

 has the following dimensions: length 16 mm., width 24 mm., 

 width of axis at anterior margin 9 mm., width of axial region 

 at narrowest point 4 mm. 



Remarks. In general form this species resembles M. brevi- 

 ceps (Hall), from the Waldron shale of Indiana, but it differs 

 from that species in several important characters. In the loba- 

 tion of the glabella M. ferrisi is much more primitive than M. 

 broyiceps, the rudimentary, secern}, lateral, glabellar furrows 

 being still discernible as slight indentations of the inner border 

 of the compound anterior lateral lobes, while in M. breviceps 

 these furrows have entirely disappeared; M. ferrisi also pos- 

 sesses a pair of third lateral glabellar lobes of .some size, while in 

 M. breviceps these have been nearly crowded out, the glabella 

 assuming in this respect nearly the condition of the genus 

 Arctinurus. These differences would v suggest at once that M. 

 ferrisi is the earlier species of the two, and this is the case, it 

 having been observed only in the lowest Niagaran beds recog- 

 nized in the Chicago area, while M. breviceps occurs in the Wal- 

 dron shale, well up towards the summit of the Niagaran. 



The species is named in honor of Mr. J. H. Ferris of Joliet, 

 Illinois, who accompanied the writer at the time the type 

 specimens were collected. 



Locality. Near Channahon, W ill Ccimty, Illircis. 



