THE NATURAL HISTORY SURVEY. 223 



Eyes situated posteriorly, oblique, the posterior extremity 

 higher than the anterior, very narrow, the length in some large 

 individuals being seven times the width, but in smaller speci- 

 mens the proportionate length is less. Posterior limb of the 

 facial suture directed obliquely outward and backward; an- 

 terior limb more than twice as long as the posterior, describ- 

 ing a slightly sigmoidal curve and cutting the anterior margin 

 outside of a line drawn tangent to the surface of the eye. Free 

 cheeks large, longer than wide, the genal angles rounded, the 

 surface depressed below the outer margin of each eye, the space 

 between the depressed surface and the lower margin of the eye 

 being occupied by a conspicuous rounded ridge nearly as con- 

 spicuous as the eye itself and parallel with it. 



The dimensions of a nearly perfect, large cephalon are: 

 length 6 1 mm., width 105 mm., convexity 46 mm., width of 

 cranidium between the eyes 83 mm. ( ' 



Remarks. This species, perhaps, has been more frequently 

 identified from the American faunas of Niagaran age, than any 

 other member of the genus, although some of these identifica- 

 tions are believed to be incorrect. The species was originally 

 described from specimens occurring in the dolomitic Niagaran 

 limestones of Wisconsin. The form commonly present in the 

 Rochester shales of the New York Niagaran, which was origin- 

 ally referred to the European species /. barriensis, is now usually 

 identified as /. ioxus, but such an identification is probably in- 

 correct; besides this several forms in the dolomites of north- 

 eastern Illinois and south-eastern Wisconsin have usually been 

 named I. ioxus by collectors, which are really quite distinct 

 species. The form described in this paper as /. chicagoensis has 

 been commonly referred to this species, but the true ^oxus is a 

 much larger and less strongly convex species, with shorter dor- 

 sal furrows, with the anterior margin of its cephalon sub-trun- 

 cate and with much narrower eyes beneath which are con- 

 spicuous ridges. The two most characteristic features of the 

 species are its large size and the sub-truncate anterior margin. 



Localities. Joliet, Illinois; Racine, Wisconsin. 

 Illaenus graftonensis M. & W., pi. xvi, figs. 4-6. 



Description. Cephalon of medium size, short, moderately 

 convex, transversely sub-elliptical in outline, somewhat con- 

 tracted posteriorly, the greatest width nearly opposite the 

 anterior extremities of the eyes, the dorsal surface rounding 

 into the ventral surface along the lateral and anterior borders- 



