401 



in six triangular points or spines, the outer ones of which are the longest 

 and the inner ones diminishing in length so as to produce a semicircular 

 emargination for the posterior outline. The greatest width of the pygi- 

 dium is at about one-third its total length from the front measuring to the 

 extremity of the longest spine. In front of a line drawn across at this 

 place the contour is nearly semicircular, but behind the line the sides are 

 straight or only gently convex, and somewhat parallel, slightly converging 

 towards each other. The main body of the axis is about one-fourth the 

 total length, convex conical and Avith four shallow concave transverse 

 grooves ; four ribs in each of the side lobes besides a rudimentary ridge 

 along the middle being a continuation of the axis. The surface is marked 

 by fine fissure-like undulating lines. 



The pleurae which seem to belong to this species are broad, flat, falcate, 

 and with a moderately strong groove running obliquely nearly their whole 

 length. 



Judging from the form of the three pygidia figured by Angelin on Plate 

 4i of the Palseontologia Scandinavica, it appears probable that this species 

 connects Dikelocephalus with Oentropleura, a genus which occurs in the 

 base of the Lower Silurian of Sweden, in Regie B and C of Angelin. It 

 is perhaps an extreme form, but the course of the facial suture and char- 

 acters of the glabella are the same as they are in Dikelocephalus. The 

 pygidium differs from D. Minnesotensis in having fewer ribs and a greater 

 number of spines, but this difference is not of itself, I think, of generic 

 value. 



Limestone No. 1. 



Dikelocephalus planiseons. (N. sp.) 



Fig. 377. 



DeseripHon. — Head with a broad smooth margin in front, the width of 

 which is about equal to the width of the glabella : the latter oblong conical, 



Fig. 377. 



Fig. 37s. 



Fig. 379. 



Fig. 377. — D. planifrons. 

 Fig. 318.— D. Belli. 

 Fig. 379. — D. Oweni. 



