MARYLAND GEOLOGICAL SURVEY 349 



on both the cephalon and pygidium. Although imperfect it agrees fairly 

 well in outline with the types of Onclwmetopus simplex. 



Occurrence. CHAMBERSBURG LIMESTONE (Nidulites bed). Wilson, 

 Maryland. Black River limestone of Minnesota and Pennsylvania. 



Collection. II. S. National Museum. 



Family ILLAENIDAE 



Genus ILLAENUS Dalman 



ILLAENUS AMERICANUS Billings 



Plate LI, Figs. 26-29 



Illaenus americanus Billings, 1859, Canadian Nat. Geol., vol. iv, p. 371. 

 Illaenus americanus Billings, 1865, Pal. Fossils, vol. i, Geol. Surv. Canada, 



p. 329, figs. 316a-d, 318. 

 Illaenus americanus Raymond and Narraway, 1908, Ann. Carnegie Mus., 



vol. iv, Nos. 3, 4, pi. Ix, figs. 1-3. 



Description. " Oblong, distinctly tri-lobed ; length two or three inches ; 

 width from three-fifths to five-sixths the length. 



" Head large, strongly convex, its height 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 ex- 

 tremities 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 



