Vol. 50.] OLENELLUS-ZOETE OF THE NORTH-WEST HIGHLANDS. 669 



long and strong. The body suddenly narrows behind the sixth 

 segment, and the pleura of the seventh and eighth segments are 

 very small; hence it is inferred that it would require only a very 

 few more segments to complete the body. Xature of telson 

 unknown. 



The characteristic features of this sub-genus are the great size of 

 the axis of the body compared with the cheeks and pleura, the 

 hexagonal head-shield with its angles set with spines, and the 

 recurrence of larger pleura and highly elongated spines on the third 

 and sixth body-segments. 



OLEXELLOIBES ARMATTJS, Sp. n0V. (PI. XXXII. figs. 1-6.) 



The head-shield, which is hexagonal and set with long spines at 

 the angles, is of about the same length as the first six body-segments 

 and varies in proportion in different individuals as 1 : 1 and 4 : 3. 

 It is bordered on five sides by a thickened, rounded margin, which 

 is widest at the angles. The anterior margin between the first 

 pair of spines is convex, and these spines are set forward at angles 

 of about 30° to 35 c to the general axis of the body. The margins 

 between these and the lateral spines make almost straight lines, 

 the head-shield being at its widest opposite the bases of these lateral 

 spines, which are set backward at angles of about 50° to the axis 

 of the body. Behind these spines the shield tapers rapidly at first 

 and then more gradually, so that the margins between the lateral 

 spines and the posterior ones are concave. The posterior spines are 

 directed backward and set at an angle of about 30° to the long axis 

 of the body ; they are slightly curved inward towards their tips. 



The posterior margin is convex, being made up of the posterior 

 margin of the occiput, which constitutes |- of the whole, the 

 remaining J- being occupied by the margins of the cheeks, which 

 are marked off from the occipital part by deep notches. The 

 glabella, which is nearly cylindrical and rounded in front, extends 

 almost from end to end of the head-shield, and occupies the greater 

 part of the cephalic area. It is divided into five lobes, wide in 

 front, narrowing somewhat at the lobe behind the first furrow, and 

 widest at the occiput. The furrows are nearly M -shaped, in con- 

 sequence of which the frontal lobe is pear-shaped and the others 

 are more or less cordate, the lobe immediately behind the first furrow 

 being the smallest. The occipital lobe bears a small spine or 

 tubercle in the mid-line. The eye-lobes are reniform, proceed out 

 from the frontal lobe towards the outer margin, and terminate just 

 behind the bases of the lateral spines opposite the third lobe of the 

 axis, the ocular slit extending throughout the greater part of its 

 outer or convex margin. The cheeks in front of the eye-lobes and 

 glabella are hollow. A narrow ridge runs from the angle made by the 

 eye-lobe and the glabella as far back as to opposite the third furrow ; 

 another ridge runs from this furrow to the base of the posterior spine. 

 These ridges are separated from the glabella and eye-lobes by deep 

 furrows. X o free cheeks nor facial suture observed. 



Body-segments, number unknown, eight only being preserved. 



