714 THE PALEONTOLOGY OF MINNESOTA. 



[Illa^nus utnericamis. 



Genns ILLJ1NUS, Dalman, 1826. 

 ILL^ENUS AMERICANUS Billings, 1859. 



Illirnus americanus BILLINGS, 1859. Canad. Nat. and Geol., vol. iv, p. 371. 

 Illcenus taurus HALL, 1861. Geol. Surv. Wisconsin; Kept. Progress, p. 4!). 

 Illcenus americanus BILLINGS, 1865. Pala-o/.oic Fossils, vol. i, p. 329, figs. 316a-d. 

 Illcenus taurus MEEK and WORTIIEN, 1868. Geol. Surv. Illinois, p. 320, pi. 3, fig. 2. 

 cf. Illcenux crassicauda (WAHL.) HALL, 1847. Paleontology of New York, vol. i, pi. 229, p. 60, figs. 

 4c, d (not 4a, b), 



This species is of frequent occurrence in various localities in Minnesota, and as 

 its dismembered parts bear certain similarities to other species with which they are 

 associated (Thaleops ovata, Bumastus trentonensis and B. orbicaudatus), it will be useful 

 to quote the very exact and detailed description given by Billings: 



Oblong, distinctly trilobed; length two or three inches; width from three-flfths to five-sixths the 

 length. Head large, strongly convex, its hight usually a little greater than its length measured on a 

 straight line, sometimes abruptly bent down at less than half the length from behind, often uniformly 

 arched from the front to the posterior margin, equal to about one-fourth of a sphere; length from front to 

 posterior margin about two-thirds the width between the cheek angles in a straight line. The glabella 

 is moderately convex; the dorsal furrows extend from one-fourth to a little more than one-third the 

 whole length of the head, measured on the curve, and have an obscure sigmoid curve, at first outwards 

 and then inwards, their anterior extremties usually turning a little outwards; they are distant from each 

 other not quite one-half the whole width of the head. The eyes are of moderate size, about two lines in 

 length, about half their length from the posterior margin, and half the width of the glabella from the 

 dorsal furrows. The cheek angles are rounded, and the posterior margin of the head makes with the 

 lateral lower margin, as seen in a side view, usually a right angle, but in some specimens an obtuse angle 

 of nearly 100, owing to the variable extent to which the front part of the head is produced downwards. 

 In some the portion of the posterior margin outside of the eye curves forwards, and brings the cheek angle 



:<> 2 



Figs. 20 23.- 1 litmus americanus (aflt:r Hillings). Fig. 20. Upper side of head. Fig. 21. Front 

 view of the same individual. Fig. 22. Pygiclinni of the same. Fig. 23. A nearly entire individual. 



to a position in front of the eye. I n others it is behind the eye. The space between the eye and the dorsal 

 furrows is convex, and the eye itself seems to be rather protuberant or subconical. The movable cheek is 

 subtriangular, its width at the posterior margin about onre and a half the distance of the eye from the 

 dorsal furrow, its length along the lower margin a little greater than its posterior width. The anterior 

 margin of the whole head is uniformly rounded, with the exception of a slight concave curve just outside 

 of the suture. In some specimens in which the front part of the head is most abruptly bent down the 

 middle portion of the front margin is depressed convex or nearly straight. 



Thorax with ten segments. Axis moderately convex, from a little more than one-third to nearly 

 one-half the width of the whole animal, a little wider at the anterior than at the posterior segment; the 

 sides sometimes straight and sometimes slightly curved outwards. On each side of the axis there is a flat 



