278 THE CHICAGO ACADEMY OF SCIENCES. 



not quite reach the lateral margins and which become obsolescent 

 posteriorly. Thorax unknown. 



The dimensions of an imperfect cephalon with the anterior 

 median projection only partially preserved are: total length 

 along the median line 24.5 mm., length of glabella from posterior 

 margin of head 15 mm., length of anterior spine so far as pre- 

 served, measured from the anterior margin of the glabella, 9.5 

 mm., total width 33 mm., height of eyes above surface of glabella 

 4.5 mm. The dimensions of a nearly complete pygidium are: 

 length 17 mm., width 20 mm., length of axis 13 mm., width of 

 axis anteriorly 7 mm. 



Remarks. This species was originally described by Hall 

 from specimens collected in the dolomitic Niagaran limestone 

 at Waukesha, Wisconsin. The type specimens, judging from 

 the illustrations, agree in all respects with the Chicago specimens, 

 but the anterior spine-like prolongation of the head is drawn 

 as a short, acuminate process. More than the broken base of 

 this process is rarely seen, and in no individual has its full extent 

 been observed. In Hall's type specimen the process was prob- 

 ably incomplete and doubtless appeared to be as his figure 

 shows, but in reality the figure is probably erroneous in this 

 particular. 



At a later date a trilobite occurring commonly in the 

 Waldron fauna of Indiana was identified with this v Wisconsin 

 species; the pygidia of the two forms are much alike, but the 

 cephalon of the Waldron form does not possess the elongate 

 anterior process, and the eyes are not nearly so highly elevated 

 above the surface of the glabella. The two forms are clearly 

 distinct, and the name D. halli has been applied to the Waldron 

 species in the bibliographic list of species in the earlier portion 

 of this Bulletin (p. 195). 



Localities. Joliet, Chicago Drainage Canal near Lemont. 



Dalmanites arkansanus van Ingen, pi. xxiv, fig. 5. 



Description. Cephalon sub-triangular in outline, bluntly 

 rounded in front, moderately convex, flattened on the axial 

 portion between the eyes, the surface sloping abruptly to the 

 lateral margins. Glabella sub-pentagonal in .outline, the an- 

 terior margin coincident with the anterior margin of the head, 

 broadest through the middle of the frontal lobe, width along 

 the posterior margin one-half the maximum width, bounded 

 laterally by shallow dorsal furrows which are somewhat deepened 

 as they approach the occipital furrow; frontal lobe large, sub- 



