399 



Thorax and pygidium unknown. I have not ascertained whether the pos- 

 terior angles of the head are rounded or produced into spines. 

 Limestone No. 1. 



Genus DIKELOCEPHALUS. Owen. 



In the species which I have referred to this genus, the general form and 

 aspect of the glabella and pygidium and the course of the facial suture are 

 the same as in D. Minnesotensis the type of the genus, specimens of which 

 I have before me from the sandstone of the Western States. From nume- 

 rous fragments of D. Oweni exhibiting the underside of the head, I have 

 ascertained that the facial suture does not separate the cheeks from each 

 other by cutting the fold of the margin. The head is therefore composed 

 of three pieces only, the glabella, hypostoma and united cheeks. This 

 separates the genus from Proetus, some species of which, such as P. stri- 

 atus (Barrande) have an expanded front margin and a spinose pygidium 

 very like those of D. magnificus. The head of Proetus consists of five 

 pieces. The hypostoma found associated with our specimens is much like 

 that of Proetus, and it is also not unlike that of Ogygia. According to the 

 figure givin in SILURIA, plate III, fig. 2, representing the sub-marginal fold 

 of the cephalic shield, and the hypostoma attached thereto of Ogygia 

 Bucliii, the structure of the head of Ogygia must be the same as that of 

 Di/celocephalus. The affinities of the two genera are still further indicated 

 by the form of the glabella. 



DIKELOCEPHALUS MAGNIFICUS. 



Description. Eight or nine inches in length. Head large with a short 

 broad obtusely conical depressed convex glabella, in front of which there 

 is a broad flat margin with from five to eight obscure radiating ridges. 

 The neck furrow is represented by an obscure shallow groove which is 

 visible in the middle two-thirds of the width, but dies out before reaching 

 the sides of the glabella. In front of this there are from one to three 

 shallow pits or faint depressions on each side of the median line represent- 

 ing the glabella furrows. A line drawn across the head at one-third the 

 length from the posterior margin would pass through the centres of the 

 eyes nearly. The eyes are annular about one-sixth the whole length of 

 the head, situated their own length from the posterior margin and with 

 their centres about the same distance from the side of the glabella. The 

 facial suture runs from the inner anterior corner of the eye forwards and 

 outwards at an angle of about 45 to the longitudinal axis of the body 



