52 Field Museum of Natural History — Geology^ Vol. IV. 



Quebec group of Utah and M. belemnura White from a similar horizon 

 in Nevada. These species were described from pygidia only, so that 

 their reference is somewhat doubtful. Judging from the descriptions 

 and figures of the above species, neither of them closely resembles the 

 pygidium of M . beckeri; so that comparison must be made with European 

 species. In general form M. beckeri most closely resembles M. ex- 

 tenuata Angelin from Gothland, Sweden, but is distinguished from that 

 species by not having the glabella defined laterally or anteriorly, by 

 having its genal spines less flaring and by having much narrower mar- 

 ginal borders of the pygidium. 



The specific name is given in honor of Mr. A. G. Becker, whose 

 collection contains the type specimen. 



Localities and horizon. — The species is known to the writer by a 

 practically complete specimen, the type, and a nearly complete cra- 

 nidium (Mus. No. P 16998). The type is from the Lower Maquoketa 

 beds at Clermont and the cranidium from a similar horizon at Post- 

 ville Junction, Iowa. 



Genus NILEUS Dalman 1826. 



"Corpus breve, convexum Iceve, sulcis dorsalibus longitudinalibus nullis; 

 segmentis trunci 8. Oculi maximi, later ales." (Palceden oder die Soge- 

 nannten Trilobiten, p. 70.) 



The above is Dalman's original description of the genus and may be 

 somewhat elaborated as follows: Body elliptical, convex, smooth. 

 Cephalon twice.as wide as long, convex, genal angles broadly rounded. 

 Glabella undefined laterally, no lateral furrows. The facial sutures 

 originate on the posterior margin of the cephalon, curve forward to the 

 eye lobes, over which they pass, thence with a sigmoid curve to the 

 anterior margin, where they meet. Eyes large, lunate, holochroal. 

 Rounded hypostoma with elevated border. Eight thoracic segments, 

 indistinctly tri-lobed, axial lobe the broader. Pygidium twice as broad 

 as long, neither lobed nor segmented, broadly rounded posteriorly. 

 Range, Ordovician of Europe and North America. 



Nileus vigilans Meek and Worthen Plate XIV, Figs. 9-15. 



Asaphus vigilans M. & W., Geol. Surv. Ills., Vol. VI, p. 497, pi. 23, 

 fig. 6. 



Nileus vigilans Clarke, Pal. Minn., Vol. Ill, pt. 2, p. 712, figs. 17-19. 



Body convex, trilobation very obscure, subelliptical in outline, 

 lateral margins nearly parallel. Surface smooth to the naked eye, but 

 under a magnifier the extremities are ornamented with transverse 



