﻿38 ME. E. S. C0BB0LD ON" TBILOBITES FROM [Feb. I9IO r 



The plane area exhibits a finely granular surface, comparable with 

 that of the cranidia ; this character continues into the base of the 

 spine, and then merges into a series of irregular raised lines, much 

 like those on the front margin of the cranidium [144]. I have 

 attempted to express these characters in the enlargements (PI. V, 

 %. 8). 



A second form of free cheek [169] (PI. IV, fig. 10) from the 

 same rock exhibits a modification, which may indicate a different 

 species : in this the spine is circular in section and much stronger ; 

 but I have other specimens somewhat intermediate between the 

 two, and, so far as the evidence goes, it would seem that the 

 setting of the spine and its sectional form are liable to variation. 



Although the free cheek of this species remains somewhat 

 doubtful, I have little hesitation in assigning the pleura?, etc. 

 [specimens 180 a, 178, 915] to it. 



The keys to the discrimination between the thoracic segments 

 of the Strenuella and the Anomocare are : — (i) the nature of the 

 surface ; (ii) the position and angle of the geniculation ; and (iii) 

 the curvature of the convexity of the axis. 



Thorax. — Number of segments unknown. 



Axis. — Transverse convexity less than a semicircle ; articulating 

 surface marked off from the body of the ring by a distinct trans- 

 verse hollow or groove ; body of the ring somewhat narrowed in 

 the middle, by the advance of the groove from the front, and by a 

 sinuosity of the posterior margin ; there is a slight cushion-like 

 thickening on each side at the point of attachment of the pleurae. 



Pleura. — Somewhat wider in the middle than at the axis; 

 nearly square with the axial liue throughout ; horizontal for a 

 distance about equal to half the diameter of the axis, then strongly 

 geniculate and bent down at an angle of about 45°; the bent 

 portion somewhat curved, convex outwards, and about a quarter 

 as long again as the horizontal part : front rib low near the axis, 

 marginal to the geniculation, whence it bends backward towards 

 and beyond the centre of the pleura, leaving a wide facet in front 

 of it ; back rib low and wide near the axis, and remaining marginal 

 throughout. 



The groove between the ribs is wide and shallow near the axis r 

 deepest at the geniculation, narrowing rapidly beyond it to die out 

 near the extremity, which is broadly rounded. 



S u r f a c e. — Finely gran ular. 



Ptgidium. — I have one fragment of a pygidium [173], with a 

 finely granular surface, that appears to belong to this species. 



Axis. — Wide; consisting of an articulating surface, two armu- 

 lations and a longer terminal portion ; the upper surfaces of the 

 annulations are broken away in a manner suggestive of their having 

 borne wide-based spines directed backwards. 



Side-lobe. — Consisting of a horizontal extension with a width 

 equal to about a quarter of that of the axis ; beyond this the 



