2 



BRITISH FOSSILS. 



posterior angles just in front of the strong curved spine, so that the 

 sides are somewhat square. The spine is about equal in length to 

 the head ; it is directed first obliquely outwards and then curved a 

 little towards the thorax again, reaching as far as to the 7th seg- 

 ment. The glabella is very convex, parallel sided or slightly para- 

 bolic if its whole contour be taken into account, but half egg-shaped 

 if the small lateral lobes are excluded ; it occupies about one third 

 the width of the head, and extends forwards only about two thirds 

 its length, a considerable though not very broad space being left 

 between it and the thickened front margin. This space, together 

 with the margin itself, about equals one third of the length of the 

 glabella. Only one pair of lobes are present, which lie at the base 

 of the glabella ; they are convex, longitudinally ovate, narrow, each 

 about one fifth the entire width of the glabella, and circumscribed by 

 a deep sulcus, which divides them as much from the glabella as from 

 the cheeks. These last are high-conical, and at about half-way up the 

 head and near the glabella, bear the large, prominent, smooth eyes, 

 which rise nearly to a* level with the highest part of the glabella ; a 

 thick margin, continuous with the front margin, surrounds the cheek, 

 and is separated from it by a strong sulcus, which does not quite 

 reach the termination of the neck furrow ; there is no abrupt hollow 

 or any depression at the angle. The facial suture, contrary to its 

 usual course in this genus, turns considerably outwards above the 

 eye to cut the front margin — along which it runs ; beneath the eye 

 its course is abruptly outwards to the end of the posterior margin — 

 a little within the base of the spine. The neck segment is tolerably 

 broad and prominent, and the neck furrow deep and straight. 



The thorax is much less convex than the head, and is always 

 a little longer than it; it consists of 11 joints, with the axis 

 moderately convex, tapering quickly backward, and of rather greater 

 width than the pleurse, especially at its anterior and posterior ex- 

 tremities ; in front about equal to the width of the glabella. The 

 6th segment of the axis is greatly swelled and produced back- 

 wards, giving rise to a straight horizontal spine, which lies upon the 

 surface of the posterior rings, and nearly reaches the end of the tail. 

 Pleurse short, flattish, divided nearly to the tip by a strong, straight 

 groove, the fulcral portion being of the same width as the posterior 

 half The ends are thickened, truncate, and very faintly bilobed ; 

 the fulcrum is placed at about half-way along the pleurse in the 

 middle segments, — at a less dista^nce posteriorly, and beyond it the 

 forward edge of the pleura is sharpened or facetted for the purpose 

 of rolling up. Tail small, transverse, and but slightly convex, much 



