Vol. 50.] OLENELI/US-ZONE OF THE NORTH- WEST HIGHLANDS. 663 



The relation of depth to breadth is nearly the same in all the 

 axes, and varies in the proportion of 1 : 3 or 3 : 8. The pleura, 

 in consequence of overlapping each other, are a little deeper 

 than the corresponding axal portions. Those of segment No. 1 

 are directed slightly forward, and preserve their full breadth as far 

 as the pleural angles of the head-shield, where they are abruptly 

 truncated, their postero-lateral angles being each set with a short 

 spine. In segment No. 2 the pleura are set at right angles to the 

 general axis of the body, and extend to the tips of No. 1 ; but 

 they are not so sharply truncated as in that segment, being bent 

 back and ending in rather longer spines. 



The pleura of segment No: 3 form a most conspicuous feature. 

 They gradually expand from the axis outward till they attain 

 double its breadth opposite the pleural spines of the preceding 

 segment, beyonot which they suddenly bend off at almost right 

 angles, taper rapidly, and are continued into more or less flattened 

 spines nearly as long as the pleura themselves. The fulcral ridges 

 and grooves are also more pronounced on this segment than on any 

 other. 



In consequence of the great expansion of the pleura of segment 

 No. 3, those of No. 4 are directed slightly backward. They are 

 much narrower and shorter than the pleura of the former segment, 

 their fulcral points being well within its posterior curves. Like 

 those of No. 1, they terminate abruptly, but their spines are a little 

 longer than in that segment. In No. 5 the pleura are set more 

 distinctly backward, and are a little longer in proportion to the 

 size of the axis than those of No. 4 ; their terminal spines also are 

 somewhat longer. 



In segment No. 6 there is a sudden increase in the size of the 

 pleura, especially in the spines, that reminds one of No. 3. The 

 succeeding four segments are all much like No. 6 in appearance, 

 though the pleura of each are set at a smaller angle to the axis 

 than the immediately preceding ones, and the spines are each in 

 turn more bent inward towards their tips in order to prevent over- 

 lapping. 



The hindermost four segments have their pleura set at an in- 

 creasingly acute angle to the axis, till the posterior margins of those 

 of the fourteenth segment almost coincide in direction with it, while 

 their spines rapidly and successively become smaller. 



The telson has not been observed in place, but it is presumed 

 that it is styliform, precisely as in the variety of this species which 

 will be subsequently described. The whole of the parts are more or 

 less marked by the peculiar reticulated pattern described in my 

 former paper {op. cit. p. 239, pi. v. fig. 2 b). This sculpture is 

 most conspicuous on the glabella and cheeks, and on the anterior 

 portion of the axes and the pleura of the body-segments. In this 

 species it is small, compared with the size of the animal. 



The chief character which distinguishes Olenellus Lapworthi from 



