718 THE PALEONTOLOGY OF MINNESOTA. 



LThaleops 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 margin of the pleura; 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 front; 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 punctae. 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 stria 1 . 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. 



Observations. The peculiar extension of the palpebra and the long, attenuate and projecting cheeks 

 are features which appeared in an earlier species, Illcenus arcturus Hall, of the Chazy limestone, and re- 

 appeared in the /. pterocephalus Whitfleld, from the Niagara limestones of Wisconsin. In the typical 

 forms of Illcenus (group of /. crassicauda Wahl.). the structural those 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/, ovatus. Mr. A. F. Foerste's /. herricki, was evidently 

 described from an entire head of I. (Thaleops) ovatus, and must hence fall into the synonymy of the 

 species. 



BUMAKTUS TRENTONENSIS EmmOHS (sp.), 1842. 



Illcenus trentonensis EMMONS, 1842. (!i:ology of New York: Ut-pl. 2d Dist., p. 390, fig. 3. 

 cf. Illcenus crassicauda (WAUL .) HALL, IT. Pahi-ontology of New York, vol. i, p. 22!: pi. 60, flgs. 4c, id. 



Illcenus milleri liiu.iMis, 185!). Canad. Nat. and Geol., vol. iv, p. 37~>. 



Illcenus milleri WALCOTT, 1879. Thirty-first Ann. Kept, N. Y. State Mus. Nat. Hist., p. 71. 

 Not Bumastus trentonensis K.MMONS. Geology of New York: Kept. Second Dist , p. 390, fig. 1. 

 Not Illcenus trentonensis HALL. Paleontology of Xew York, vol. i, p. 230; pi. 60, llg. .",. 

 Not Illcenus crassicauda HALL. he. cit., pi. 60, flgs. 4a, 46. 



