414 ME. F. E. COWPEE EEED ON THE [A tig. 1 896, 



identifying the two. A badly preserved pygidium of an lllcenus 

 from Keisley also shows the characters of that part as described 

 and figured by Holm. 



Measurements. 



millim. 



Length of middle shield 170 



Width (greatest) of do 305 



Length of glabella 7'0 



Width of base of do 8-0 



Length of pygidiuin 15*0 



Width of do 29-0 



Width of front end of axis 7*0 



Ill^nus galeatus, sp. n. (PI. XX. figs. 1, 2 & 3.) 



A dozen or more specimens of an lllcenus in the Woodwardian 

 Museum, which have their head-shields inflated to an extraordinary 

 extent, possess a curious combination of characters. There is a great 

 conical elevation situated nearly in the centre of the middle shield, 

 and from its summit the surface slopes down regularly to the margins, 

 but more steeply to the back and front than to the sides. Billings's 

 species I. conifrons from the Chazy Limestone of Canada l resembles 

 these Keisley forms in the abnormal amount of inflation, and 

 Prof. Nicholson, who presented us with some of our specimens 

 from Keisley, was accordingly led to compare them. 2 But in reality 

 these British individuals are widely separated from the Canadian 

 species by several essential structural features which are pointed 

 out below. 



The description of the Keisley form is as follows : — 



The middle shield is nearly semicircular in front ; its posterior 

 margin is almost straight ; the width is about half as much again 

 as the length. It slopes up regularly from all sides to a subcentral 

 conical elevation, the summit of which is obtusely rounded and 

 situated immediately in front of the glabella. The profile of the 

 middle shield across its middle, seen from behind, is like the small 

 end of a fowl's egg ; but viewed from the side the profile is semi- 

 elliptical. The posterior slope is a little steeper than the anterior, 

 and these two slopes are more convex than the lateral slopes, the 

 latter being decidedly flattened. The vertical height of the middle 

 shield is about half its greatest width. 



The glabella is short, occupying only about two-thirds of the 

 length of the posterior slope. It is about one and a quarter times 

 as long as wide. It is almost rectangular and parallel-sided in 

 shape ; but it suffers a very little contraction in the middle through 

 the gentle inward curvature of the axal furrows. It is undefined 

 in front, but is bounded posteriorly by a shallow, though distinct 

 neck-furrow which is arched slightly forward and marks off a 

 narrow occipital ring. The glabella has a convexity independent 



1 Billings, 'Canadian Naturalist and Geologist,' vol. iv. (1859) p. 378; and 

 Rep. Progr. Geol. Surv. Canada, 1863, ch. ix. p. 151, fig. Ill, a, b. 



2 Marr and Nicholson, ' The Cross Fell Inlier,' Quart. Journ. Geol. Soc. 

 vol. xlvii. (1891) p. 507. 



