﻿Vol. 66.~] THE CAMBRIAN ROCKS OF COMLEr. 3B 



some faint and very slightly raised lines, running longitudinally, 

 and on one front border [945] there are similar marks almost on 

 the under surface. 



Free cheeks. — Among the innumerable fragments associated 

 with these cranidia and with other similar fragments belonging to 

 the next species to be described, there are several free cheeks which 

 I feel confident may be assigned to this species. 



One of the most perfect [170] is figured in four aspects (PI. IV, 

 figs. 6 a-6 d) : fig. 6 a is a representation of the cheek as it lies 

 flat in the rock ; fig. 6 b is the view presented as seen from the side 

 of the complete organism ; fig. 6 c is the view presented as seen from 

 above (as nearly as I can judge) ; fig. 6 d is an enlargement of the 

 spine, to show the surface-markings. These markings correspond 

 very closely with those on the spine and front border above men- 

 tioned, but they are more pronounced. They consist of raised, 

 inosculating, flattened lines, looking like tiny ribbons of gossamer, 

 irregularly arranged subparallel with the margin. The rest of the 

 surface is of exactly the same texture as the associated cranidia. 



The transverse curvature of the cheek-margin agrees with that 

 of the cranidiurn, and the lower branch of the facial suture on the 

 cheek would (with allowances for differences in size and individual) 

 fit fairly well to the suture shown on the head-shield. 



The flat area of the cheek is very small ; the rise to the eye is 

 steep and pronounced ; the marginal fold is produced at the genal 

 angle into a stout spine, somewhat oval in section, and this projects 

 (to judge by fig. 6 c) but slightly from the general outline of the 

 thorax. 



The associated thoracic segments that maj T be referred to this 

 species are represented in PI. IV, figs. 2-4 and 7-9. They are 

 characterized by a very convex axis usually, if not invariably, more 

 than a semicircle ; and by a pleural lobe which is horizontal for half 

 its length, and has a strong geniculation at a distance out subequal 

 to the transverse diameter of the axis, the tip being bent down at 

 an angle of 60° to 65°. In some axis-rings there is a very small 

 central tubercle ; others are provided with the base of a stout 

 spine directed vertically upwards, and seen, in one instance [176] 

 (fig. 4), to rise fully three-quarters of the height of the ring without 

 change of direction. The extremities of the pleurae are rarely easy 

 to observe : in [162] fig. 3 the whole tip is corroded and inde- 

 finite ; in [164] fig. 2, which was carefully excavated with a needle, 

 the tip appears to be distinctly rounded ; in [180] fig. 7 a distinct 

 but somewhat incipient hook is observed ; in [944] fig. 8 the hook 

 is more pronounced ; and in [948] fig. 9 a much stronger hook is 

 seen. The last two are detached tips unconnected with any axis- 

 ring, but showing an angle of geniculation of about 60°. 



Summarizing from the above-quoted specimens, the characters of 

 the thorax may be described as follows : — 



Thorax. — Number of segments unknown. 



Axis. — Very convex ; anterior segments plain, or provided with 

 Q. J. G. S. No, 261. d 



