728 THE PALEONTOLOGY OF MINNESOTA. 



I Pterygometopns 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. iz.Pterygomttopus in- Fig. 46. Profile of the same. Fig. 47. Pygidiuni 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, not a spine. The lateral 

 slopes of th.e 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 pleurae. 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): Chatfleld; Trenton: Lake Street 

 Bridge, St. Paul; Minneapolis; Galena: Wykoff, Minnesota. 



PTEBYGOMETOPUS EBORACEUS, n. 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 



Fig. 48. Pterygometopus 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 



