420 REPORT ON THE STATE CABINET. 



single specimen partially preserving the crust does not show the glabellar 

 furrows so distinctly as the casts. 



" In fig. 10, the head is represented showing the dorsal furrow and direc- 

 tion of the facial suture, and in fig. 11, a profile view is given of the same 



I received several years since a specimen of this species from Prof C. 

 U. Shepard, who collected it with other Niagara fossils in Illinois, but 

 the record of the particular locality had been lost. 



Formation and Locality. — In the limestone of the Niagara group at 

 Waukesha and Milwaukee, Wisconsin, and from a similar horizon in 

 Illinois. 



Ill^nus imperator. Hall. 



PLATE XXII, FIGS. 15-17; PLATE XXIII, FIGS. 2, 3. 

 Illmnus imperator, 'Ra.ia,. Report Progress Geol. Survey Wisconsin for 1860, p. 49. 1861. 



This species, which was originally described from some large caudal 

 shields with a few of the articulations of the thorax, has proved to be not rare. 



The head is large and broad, moderately convex, and pretty regularly 

 arching from the base to front ; the glabella occupies about one-third the 

 entire width ; dorsal furrows wide, extending about half the entire length 

 of the head, and curving outwards at the anterior extremity. 



One large head has a length of three inches and a half, with a width 

 between the facial sutures of four and a quarter inches. The eyes and 

 cheeks are but partially known. 



The caudal shields present gradations in size, from a length of half an 

 inch by a width of seven-eighths of an inch, to those of less than three 

 inches long with a width of four and a half inches. The proportions of 

 length and breadth of the pygidium are not constant, though its wide 

 and very depressed form is always characteristic. 



The position of this species is somewhat lower in the group than the 

 Racine and Waukesha beds. 



ILL.ENUS (BUMASTUS) lOXUS, HaLL. 



PLATE XXII, FIGS. -1-10. 



Compare Bumastus barriensis, Mukchison. Silur. Syst., p. 656, PI. vii, bis, fig. 3a-d. 1839. 

 Bumastus barriensis, Hall. Palaeontology N. Y., II, p. 302, Plate Ixvi, figs. 1-15. 



There are several slight differences between the American (Wisconsin) 

 specimens referred to this species, and the figures and descriptions of the 

 English form of I. barriensis, as given in the British Decade 2, Plates 3 



