﻿Tol. 66.~] TRILOBITES FROM THE CAMBRIAN ROCKS OE COMLEI. 21 



Microdiscus. 



Microdiscus comleyexsis, sp. nov. (PI. Ill, figs. 1-4.) 



E. S. Cobbold, Microdiscus sp., Kep. Brit. Assoc. 1908 (Dublin) p. 236 (1909). 



This species is founded upon the examination of over seventy 

 ihead-shields and pygidia, the thoracic segments being unknown. 



Cephalox : Size — moderate ; length = from 3 to 4 millimetres. 



General form.— Semi-elliptical ; proportion of length to width 

 about 3 to 4; evenly rounded at front and sides, and having a 

 sinuous posterior border ; postero-lateral angle gently rounded. 



General convexity. 1 — Considerable, about \. 



Glabella.— About seven-eighths of the total length and a third 

 ■of the width of the shield ; subcorneal, widest at the posterior end, 

 which is also somewhat swollen and campanulate ; anterior end 

 semicircularly rounded, and reaching to the flattened marginal rim ; 

 posterior end, with which the occipital ring is entirely confluent, 

 rounded elliptically, and projecting considerably beyond the pos- 

 terior margin of the shield ; no transverse furrows. 



Axial furrows. — Sharply impressed; wider in the internal 

 cast than on the exterior. 



Cheeks. — Gently and evenly convex; greatest elevation, about 

 half the height of the glabella at a point rather in advance of the 

 middle of their length ; the exterior boundary is nearly a quadrant 

 •of a circle, which is tangential to the apex of the glabella. 



Postero-lateral border. — A somewhat sinuous rim, raised 

 ■(? geniculate) at a point about three-fourths of the width outwards. 

 At the posterior angle there is a little depression crossing the 

 margin, and separating the side marginal rim from the postero- 

 lateral border. 



Marginal rim. — Standing out horizontally from the cheeks, 

 and then bent downwards ; rather narrow at the sides ; widening 

 to about an eighth of the length of the head-shield in front of the 

 glabella ; without tubercles 2 ; having no groove in its rear. 



Pygidium: General form. — Approximating very closely to 

 that of the head-shield, but rather more convex. 



Axis. — Tapering regularly backwards and extending the full 

 length of the shield ; anterior end projecting a little beyond the 

 general line of the margin with a narrow articulating rim (rarely 

 preserved) ; posterior end rounded, and having the appearance of 

 being pressed down into the confluent swollen side-lobes ; divided 

 into about seven annulations, hardly discernible on the exterior, 

 but sometimes seen in the cast of the interior [508] :3 (PI. Ill, fig. 4). 



1 The fractions used under this heading, in this and the following descrip- 

 tions, to express the convexity, indicate the proportion borne by the greatest 

 height of the head-shield to its transverse width. 



2 When viewed sideways, some of the specimens exhibit a very slight 

 irregularity in the horizontal surface of the marginal rim towards the sides of 

 the head-shield, suggestive of incipient (or decadent) tubercles. They are so 

 badly defined, that they can neither be counted nor shown on the figures. 



3 The numbers in square brackets are those attached to the specimens 

 "themselves. 



