716 THE PALEONTOLOGY OF MINNESOTA. 
{Illzenus indeterminatus. 
what abraded and in rather an unfavorable’ condition for study. An enrolled 
specimen from Kenyon retains the parts better than any other observed. 
Formation and locality.—Illenus americanus, like I. crassicauda, has a very restricted vertical 
range, though of distinctly later date than the latter. Billings speaks of it as a rare species occurring in 
the “Trenton limestone only,” at Ottawa, L’Orignal, and lake Huron. In the Trenton limestone of 
Trenton Falls it is not uncommon and is exquisitely preserved. In Minnesota it is kncwn only in the 
Galena limestone and shales at Wykoff, Kenyon, Old Concord, Cannon Falls, and in Goodhue county; 
also at Oshkosh, Wisconsin, and Galena, Lllinois. 
ILLanus; compare I, 1npETERMINATUS Walcott. 
Tilenus indeterminatus WALCOTT, 1879. Thirty-first Ann. Rept. N. Y. State Mus. Nat. Hist., p. 70. 
Fig. 24.—Cranidium of Illenus; ef. I. indeterminatus Walcott. 
There is a single cranidium of comparatively large size, from the lower Trenton 
beds at Janesville, Wisconsin (Museum No. 8413), which agrees very well with the 
description given by Walcott, and is characterized by the conspicuous development 
of the dorsal furrows, which clearly define the lateral outline of the glabella. Mr. 
Walcott’s original specimens were from Herkimer county, N. Y. (Black River lime- 
\ 
stone), and from Plattesville, Wisconsin. 
Subgenus THALEOPS, Conrad, 1843. 
TuaLrors ovata Conrad, 1843. 
Thaleops ovata CONRAD, 1843. Proc. Acad. Nat. Sci. Phil., vol. i, p. 332. 
Thaleops (Illenus) ovatus HALL, 1843. Paleontology of New York, vol. i, p. 259; pl. 67. figs. 6a, b. 
Illeenus ovatus WHITFIELD, 1882. Geology of Wisconsin, vol. iv, p. 288; pl. 5, figs. 1-2. 
Illeenus herricki FoERSTE, 1887. Fifteenth Ann. Rept. Geol. and Nat. Hist. Surv. Minnesota, p. 
479, fig. 2. 
This appears to be the most abundant of the Minnesota trilobites; and though I 
have seen but two essentially entire specimens, separated heads and tails are of 
frequent occurrence. The species is very characteristic in its structure and was 
clearly described by Mr. Conrad from entire individuals. The diagnostic features 
indicated by him, and which lead at once to the identification of the species, are the 
deep lobation of the cephalon, the attenuate cheeks, divergent, tapering, peduncular 
eye-nodes, and the complete isolation of the axis of the pygidium. The first of 
these features varies more or less and is better defined on internal casts than on the 
external surface. 
