714 THE PALEONTOLOGY OF MINNESOTA. 
{Ille#nus americanus. 
Genus ILLAINUS, Dalman, 1826. 
Ittanus AmERIcANUS Billings, 1859. 
Illenus americanus BiLLINes, 1859. Canad. Nat. and Geol., vol. iv, p. 371. 
Tilenus taurus HALL, 1861. Geol. Surv. Wisconsin; Rept. Progress, p. 49. 
Illenus americanus BILLINGS, 1865. Paleozoic Fossils, vol. i, p. 329, figs. 316a-d. 
Tllenus taurus MEEK and WORTHEN, 1868. Geol. Sury. Illinois, p. 320, pl. 3, fig. 2. 
cf. Illceenus crassicauda (WAHL.) HALL, 1847. Paleontology of New York, vol. i, pl. 229, p. 60, figs. 
4c, d (vot 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-fifths 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 thefront 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 
20 : zz 
Figs. 20—23.-—Illenus americanus (after Billings). Fig. 20.—Upper side of head. Fig. 21.—Front 
view of the same individual. Fig. 22.—Pygidium of thesame. Fig. 23.—A nearly entire individual. 
to a position in front of the eye. In others it is behind theeye. 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 once 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 aninial, 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 
Fel f ae ee ae ‘ “3 
Paps] eae eee Pt eee ere, 
4 aan) ey Se 
jae ea ae 
x. 
7 a ee 
af 
Tesh, soa -* 
olf 
