THE NATURAL HISTORY SURVEY. 239 



cranidium between the eyes 18 mm. The dimensions of a pygid- 

 ium are: length 12.5 mm., width 17.5 mm. 



Remarks. This species, founded on the pygidium alone, 

 was briefly described by Winchell and Marcy from Bridgeport. 

 Their illustration was clearly incorrect, as pointed out by them 

 in the "errata" of their paper, and really .does not represent the 

 essential characters of the species at all. The species, however, 

 is not uncommon in the collections from Bridgeport, it is in fact 

 the most common lichad which has been observed from that 

 locality. Hall did not recognize Winchell and Marcy's species at 

 all, but figured a pygidium (Rep. N. Y. St. Cab. Nat. Hist., pi. 21, 

 fig. 14) from Bridgeport under the name Lichas breviceps, whicfc 

 he afterwards made the type of the new species Lichas emargi- 

 natus, which represents the species far better than Winchell 

 and Marcy's original illustration. The head figured by Hall in 

 the same place differs from anything that has been observed 

 during the preparation of the present Bulletin, and doubtless 

 belongs to another species. The head which is here associated 

 with this species occurs about as commonly in the Bridgeport 

 collections as the pygidia, and while these two portions of the 

 body have never been found in such a position as to indicate 

 that they originally belonged to the same individual, yet the 

 heads and pygidia are not infrequently found closely associated. 

 Occurring as they do in this manner, in approximately equal 

 numbers, there seems to be no reasonable doubt as to the cor- 

 rectness of considering them to be parts of one species. If 

 they do not belong to the same species, then there is no head for 

 the common pygidium and no pygidium for the common head. 

 The cephalon which is illustrated herewith on plate xxii, is not 

 from Bridgeport, but from near Lemont, it differs from the 

 Bridgeport specimens in its larger size and in the more sparse 

 tuberculation of the surface, it has been used for illustration 

 because it is more complete than any of the Bridgeport speci- 

 mens in that it preserves one of the free cheeks. 



The species which has recently been described as Arges 

 arkansana by van Ingen, from the St. Clair limestone of In- 

 dependence County, Arkansas, is very closely allied to this form 

 from the Chicago area, and it is possibly not distinct from it. 

 Both of these species, if they are distinct, are somewhat closely 

 related to the Bohemian species Lichas scabra Beyr,, the Amer- 

 ican and the European forms being co-generic. 



Localities. Bridgeport, Hawthorn and Chicago Drainage 

 Canal near Lemont. 



