Vol. 50.] OLENELLUS-ZONE OP THE NORTH-WEST HIGHLANDS. 



667 



placed in the genus Olenellus. As it combines several of the features 

 of the sub-genera of Olenellus, I have given it the specific name of 

 intermedins. 



Fig. 1. — Olenellus </igas, sp. nov. (Natural size.) 



The head-shield is roughly hexagonal, the anterior and posterior 

 sides of the hexagon being much the longest, so that the shield is 

 wide compared with its depth. The anterior margin, which forms 

 three sides of the hexagon, is divided into a long median section, 

 which is slightly convex, bent round at obtuse angles and con- 

 tinued into the other two sides ; these are straight and produced 

 backwards into strong genal spines, the whole margin being 

 strengthened by a well-rounded ridge. The posterior margin makes 

 up the other three sides of the hexagon. The median portion, 

 made up of the margin of the occiput and the edge of the cheeks 

 as far as the pleural angles, is convex. At the pleural angles 

 are placed two spines (Ford's ' interocular spines '). The margin 

 between these and the genal spines is defined by almost straight lines, 

 which form the remaining sides of the hexagon. It is thickened 

 in the same manner as the anterior margin. 



The cheeks are tumid and divided from the raised margin, the 

 glabella, and the eye-lobes by a depression. A groove, continuous 

 with that of the occipital groove of the glabella, is produced out 

 towards the base of the spine upon the pleural angle. No free 

 cheek nor facial suture has been observed. The glabella, which is 

 highly embossed, extends from close upon the thickened anterior 



