TRILOBITRS. 713 
Nileus vigilans.]” 
margin. Facial sutures making a broad outward curve on their anterior limb, 
incurving again near the anterior margin, reaching it in front of the center of the 
eyes and traversing the margin without angulation; posterior curves intersecting 
the posterior margins at large angles not far from the genal extremities. 
Fig. 19.—Nileus vigilans Meck and Worthen. An entire individual, with cephalon inclined. 
Thorax composed of eight broad, flat segments. Longitudinal lobation very 
obscure, axis very broad, covering three-fourths of the entire width of the thorax. 
The segments are broad in the middle, narrowing somewhat at the axial furrows; 
on the pleura they are very narrow, not grooved, and the beveled articulating 
planes extend for the entire distance from the dorsal furrows. 
Pygidium comparatively short, subsemicircular on the margins. Surface 
smooth, sloping equally to the sides and posterior extremity, slightly concave just 
within the margin; without external evidence of lobation or segmentation, On the 
cast of the inner surface the axis is seen to be considerably narrower at the anterior 
margin than at the termination of the thorax, and its lateral margins taper regularly 
toa point not distant from the posterior extremity of the shield; in a favorable 
light eight annulations may be counted on the axis and five on the pleura. The 
articulating ring and groove on the anterior margin are broad and conspicuous. 
Doublure broad, coarsely striated as in Isotelus. Surface ornamented, especially on 
the extremital portions, by coarse venation traversing the test transversely. 
A very young entire example has a length of 16 mm.; the largest extended 
specimen is 26 mm. in length, and the largest enrolled example 50 mm. in length. 
It is evident from certain isolated pygidia that these dimensions were frequently 
exceeded. 
Formation and locality.—Lower Trenton (Black River), Minneapolis; Galena shales, Wykoff, 
Pleasant Grove, Minnesota. The species was originally described from the Hudson River fauna of 
Carroll and Kendall counties, Illinois. 
Observations—This species resembles in many respects the Bumastus trentonensis, with which itis 
associated; but it will be readily distinguished therefrom by the position of the eyes, form of the facial 
sutures, shallowness of the cephalon, and great breadth of the thoracic segments. Normal forms of the 
genus Nileus are characterized by the great length of their eyes. This feature is seen in N. armadillo 
Dalman, the type of the group, in NV. palpebrosus Angelin, and in the three species described by Billings 
from the Quebec group, N. scrutator, N. macrops, N. affinis.* These long, sublunate eyes are forcible 
evidence of morphological immaturity, which is corroborated by the earlier age of such forms of the 
genus, Diminution in strength and increase in hight of the eyes, as in N. vigilans is undoubtedly 
the accompaniment of phyletic maturity in this group. Hence I have felt no hesitation in endorsing Mr. 
Foerste’s generic reference of this fossil; though if another name were current, it might be useful as 
indicating the different stage of development attained by the later forms, 
* Paleozoic Wossils, vol. vil, 1865, pp. 273-275. 
