THE NATURAL HISTORY SURVEY. 233 



of the head from the occipital furrow; frontal lobe of the gla- 

 bella not differentiated laterally from the cheeks ; a single pair 

 of lateral glabellar furrows which at their inner extremities 

 curve backward and join the occipital furrow, cut off a pair 

 of sub-ovate lateral glabellar lobes; occipital furrow not 

 continuous across the median portion of the glabella; occipital 

 segment rather broad, with a pair of small occipital lobes back 

 of the postero-lateral margins of the glabellar lobes; palpebral 

 lobes comparatively broad and moderately elevated. 



Pygidium sub-semielliptical or parabolic in outline, wider 

 than long; axis short, sub-semielliptical in outline, with one or 

 two transverse furrows near the anterior margin; upon each side 

 the limb is marked by seven slightly elevated ribs which become 

 obsolete before reaching the margin, the anterior ones usually 

 have a slight sigmoidal curvature; the median rib or post-axial 

 region is about twice as wide as the lateral ribs, through the 

 first half of its length from the posterior margin of the axis it 

 increases gradually in width, this increase becoming more rapid 

 posteriorly until at the margin it is four or five times the width 

 of its point of origin. 



The dimensions of a nearly perfect pygidium are: length 

 33.5 mm., width 41 mm. 



Remarks. The heads of this species are rarely preserved, 

 and the few specimens which have been observed are in a yery 

 fragmentary condition. The above description of this portion 

 of the species has been taken from Hall's illustration and defi- 

 nition. The pygidia also are among the rarer members of the 

 fauna of the Chicago area, although they are commonly much 

 better preserved than the heads. 



Winchell and Marcy's species B. occasus has been shown 

 by Hall to be a synonym of B. acamas, although it would be 

 impossible to determine this from the original description and 

 illustration of the species. Hall, however, has refigured the 

 type specimen of B. occasus and this corrected drawing shows it 

 to be only a small specimen of B. acamas. 



Localities. Bridgeport and Romeo. 



Family 5. LICHADIDAE Barrande. 



"Dorsal shield generally large and flat, with granulated 

 test. Cephalon small, not more than one-fourth the entire 

 length; genal angles spiniform. Free cheeks separate; sutures 

 extending from the posterior margin obliquely inward to the 

 eyes, and then almost directly forward, cutting the margin 



