716 THE PALEONTOLOGY OP MINNESOTA. 



[Illanius indetermin:iUis. 



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. Illcenus americanus, like /. 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 known only in the 

 Galena limestone and shales at Wykoff, Kenyon, Old Concord, Cannon Falls, and in Goodhue county; 

 also at Oshkosh, Wisconsin, and Galena, Illinois. 



ILL^ENUS; compare I. INDETEBMINATUS Walcott. 



IttcenuB indeterminatus WALCOTT, 1879. Thirty-first Ann. Kept. N. Y. State Mus. Nat. Hist., p. 70. 



Fig. 24. Cranidium of Illcenu; cf. 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. 

 THALEOPS OVATA Conrad, 1843. 



Tlutleops ovata CoNitAD, 1843. Proc. Acad. Nat. Sci. Phil., vol. i, p. 332. 



Thaleops(Illcenus)ovatus HALL, 1843. Palaeontology of New York, vol. i, p. 259; pi. fi7. flgs. 6a, b. 

 Illtenus ovatug WIIITKIKLD, 1882. Geology of Wisconsin, vol. iv, p. 238; pi. 5, flgs. 1-2. 

 Illtenus herricki FOEKSTE, 1887. Fifteenth Ann. Kept. Geol. and Nat. Hist. Sur.v. Minnesota, p. 

 479, Ilg. 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. 



