﻿34 MR. E. S. COBBOLD ON IRILOBITES FROM [Feb. I9IO, 



a central tubercle ; median and posterior segments all (?) provided 

 with stroDg cylindrical spines, directed vertically upwards ; arti- 

 culating surface marked off from the body of the ring by a decided 

 transverse hollow. At each side of the axis there is a small 

 cushion - shaped thickening, perhaps serving to strengthen the 

 attachment to the pleura. 



Pleura. — Standing out nearly at right angles to the axial line ; 

 horizontal for a distance subequal to the transverse diameter of 

 the axis ; then suddenly geniculated and bent downwards at an 

 angle of 60° to 65° ; the bent portion is subequal in length to the 

 horizontal portion, and slightly convex outwards ; the extremities 

 are rounded or armed with incipient hooks, which, possibly, become 

 more pronounced posteriorly. The surface of each pleura is pro- 

 vided with a front and a back rib, with a furrow between them : 

 the back rib is marginal throughout ; the front rib is marginal to 

 the geniculation, from which point it bends a little backwards, 

 leaving a narrow facet in front of it, and causing the furrow to 

 contract in width, until it disappears at a distance from the extremity 

 equal to about the width of the pleura; where a hook is present, 

 it is formed of a continuation of this front rib carried backwards. 



Surface. — Smooth or very finely granular, with a slightly 

 coarser granulation at the extremities. 



A fragment [274] of a pygidium (PI. IV, fig. 5) may be referred 

 to this species with a high degree of probability. The surface 

 characters agree with those above described ; the sides and poste- 

 rior margin are bent down at a steep angle ; the general form is 

 very like that of the pygidium figured by Walcott * as associated 

 with the head-shields which he refers to Agraulos strenuus, var. 

 nasutus. The one doubtful point is the flattened upper surface 

 of the axis, which does not exhibit the high convexity of the 

 glabella and thoracic segments. 



Pygidium. — The following is the description of the associated 

 pygidium referred with very little hesitation to this species. 

 General form. — Trapezoidal; wider behind than in front. 

 Axis. — Convex; somewhat flattened in the middle; reaching 

 the posterior end of the shield ; with no divisions except that 

 marking off the articulating surface, which projects a little beyond 

 the general line of the anterior border. 



Side-lobes. — Subtriangular ; not confluent behind the axis; 

 front border standing out at right angles to the axis for one-third 

 of its length and then bent suddenly backwards and slightly down- 

 wards to meet the posterior margin at a rounded angle ; posterior 

 margin almost straight and bent down vertically. In the view 

 from behind, this hanging portion is seen to arch suddenly upwards 

 as it approaches the axial line. 



The surface of the side-lobe slopes almost uniformly backwards 

 and outwards from the inner anterior angle, and is marked by 



1 10th Ann. Eep. U.S. Geol. Surv. 1888-89 (1890) p. 653 & pi. xcvii, figs. 1, 



la-c. 



