728 THE PALEONTOLOGY OF MINNESOTA. 
[Pterygometopus eboraceus. 
but clearly defined, not confluent with the second lobes at their outer margin. The 
occipital ring is moderately broad, considerably elevated and without evidence of 
spine or central tubercle. At the angles of the cheeks are short, sharp spines, not 
extending beyond the second thoracic segment. The lateral margins of the cheeks 
are bordered by a thickened rim. 
Fig. 45.—Plerygometopus in- lig. 46.—Profile of the same. Fig. 47.—Pygidium of another 
termedius Walcott. Cephalic view individual. x 2. 
of an enrolled individual. x 2. 
Although the thorax is preserved in several very neat, enrolled specimens, there 
is nothing of diagnostic value to add to the description already given. It tapers 
more rapidly than is usual in the later dalmanitids, but by no means as much so as 
in Dalmanites achates Billings. 
The pygidium is eminently triangular in marginal outline, the sides making a 
sharp posterior angle, which, when the animal is enrolled, projects conspicuously 
beyond the anterior margin of the head. This is, however, nota spine. The lateral 
slopes of the pygidium are decidedly abrupt, especially toward the posterior 
extremity. Our specimens agree with those described by Mr. Walcott, in having 
from 10 to 14 (when clearly retained) annulations on the axis, and 8, with some- 
times traces of 2 more, ribs on the pleure. The first three or four of the latter are 
usually faintly sulcate. The surface of the glabella is very sparsely tubercled; usually 
only traces can be seen, and hence the generally smooth aspect of the head. 
Formation and locality.—Lower Trenton (Black River horizon); Chatfield; Trenton: Lake Street 
Bridge, St. Paul; Minneapolis; Galena: Wykoff, Minnesota. 
PrERYGOMETOPUS EBORACEUS, 2. Sp. 
This is a New York form, closely allied to the foregoing species, though 
differing from it in some interesting structural details. The glabella is of the same 
Hig. 48.—Plerygometopus eboraceus. 
general form in both; but here the median portion between the first and second 
lobes is decidedly convex and scarcely depressed, The first, second and third lateral 
