718 THE PALEONTOLOGY OF MINNESOTA. 
} [Thaleops ovata. 
Pygidium short, transversely subquadrate, the posterior margin being a very 
broad curve; width equal to nearly twice the length. Axis very prominent, much 
narrower than at the termination of the thorax. It tapers very gradually and 
Fig. 26.—Pygidium of Thaleops ovata Conrad. Fig. 27.—The same viewed from behind. 
terminates bluntly in an elevated extremity, which is faintly bilobed. The axis is 
thus entirely surrounded by the dorsal furrows. Its length is about one-half that of 
the pygidium and its extremity lies at the beginning of the convex posterior 
deflection of the shield. Anterior mafgin of the pleure straight for one-half their 
extent, thence deflected at nearly right angles. Surface flat above, curving abruptly 
to the margins. All annulation of the pygidium is very faint, but in well preserved. 
Fig. 28.—Thaleops ovata Conrad. Outline of head viewed from the frcnt; showing the terete genal 
extremities. From an impression of the external mould of the specimen shown in figure 25. 
specimens under favorable illumination, traces of five may be counted on the axis 
The surface of the cephalon is covered with epidermal puncte. On the cheeks and 
over the anterior portion of the glabella these are vertical and isolated; over the 
posterior surface of the glabella they become oblique and confluent, making an 
irregular series of elevated anastomosing strie. The segments of the thorax 
appear to be quite smooth. On the pygidium, especially over the anterior portion of 
the axis, the punctations are deep, coarse, and arranged in transverse rows. 
Formation and locality.—Trenton limestone, Minneapolis. Minnesota; Beloit, Janesville, Mineral 
Point, Wisconsin; Dixon, Rockton, Illinois; Decorah, Iowa. s 
Observations.—The peculiar extension of the palpebra and the long, attenuate and projecting cheeks 
are features which appeared in an earlier species, Il/lenus arctwrus Hall, of the Chazy limestone, and re- 
appeared in the J. pterocephalus Whitfield, from the Niagara limestones of Wisconsin. In the typical 
forms of Illcenus (group of I. crassicauda Wahl.), the structure of these parts is so different (low, sessile 
eyes and broad, obtuse, unprojecting cheeks) that Conrad’s term Thaleops may well be retained for the 
subordinate type of structure represented by JZ. ovatus. Mr. A. F. Foerste’s I. herricki, was evidently 
described from an entire head of Z.(Thaleops) ovatus, and must hence fall into the synonymy of the 
species. 
BuMASTUS TRENTONENSIS Hmmons (sp.), 1842. 
Illeenus trentonensis EMMONS, 1842. Geology of New York; Rept. 2d Dist., p. 390, fig. 3. 
ef. Illenus crassicauda (WAHL.) HALL, 1847. Paleeontology of New York, vol. i, p. 229; pl. 60, figs. 4c, 4d. 
Tlleenus milleri BILLINGS, 1859. Canad. Nat. and Geol., vol. iv, p. 375. 
Tllenus milleri WALcort, 1879. Thirty-first Ann. Rept., N. Y. State Mus. Nat. Hist., p. 71. 
Not Bumastus trentonensis }MMons. Geology of New York; Rept. Second Dist , p. 390, fig. 1. 
Not Tilceenus trentonensis HALL. Paleontology of New York, vol. i, p. 230; pl. 60, tig. 5. 
Not Jllenus crassicauda HALL. loc, cit., pl. 60, figs. 4a, 4b, 
