MICRODISCUS SCULPTUS. 35 



4. Microdiscus sculptus, Hicks. Plate III, figs. 9, 10. 



1871. Microdiscus sculptus, Hicks, Quart. Journ. G-eol. Soc, vol. xxvii, p. 400, pi. xvi, figs. 9, 

 9 a, 10, 10 a. 



Head semicircular or semi-elliptical, marginate. Glabella conical, about one 

 third as wide and rather more than half as long as the head, bounded by 

 well-marked axial furrows. Cheeks about as wide as the glabella posteriorly, 

 nearly uniform in width, separated in front of the glabella by a groove ; occipital 

 furrow deep. Margin very broad in front, narrowing rapidly to the genal angles, 

 ornamented in front by a row of rather large tubercles. 



Thorax of three segments. Axis forming about one third of the width. 

 Pleurae deeply grooved, bent downwards at the tips. 



Tail semicircular or semi-elliptical, marginate. Axis about a quarter the width 

 of the whole tail in front, prominent, conical, not quite reaching to the margin, 

 divided into six rings besides the articular ring and the terminal portion, some of 

 the rings with a pair of small tubercles. Lateral lobes with five distinct furrows 

 separating rounded ribs ; the first furrow nearly at right angles to the axial line ; 

 the last very close to the axis and almost parallel with it. Margin narrow, flat. 



Total length of entire but compressed specimen, 8*5 mm. ; width 3 mm. 



The only specimen known which shows the head and thorax is, unfortunately, 

 very imperfectly preserved, and is, apparently, much compressed. In an undis- 

 torted specimen the outline of the head and tail would probably be semicircular. 

 It is impossible to say whether a neck-spine was present, and very difficult to 

 determine the number of thoracic segments. Hicks's figure shows four; but in 

 front of the tail there are only four grooves, and of these the first, I think, is the 

 occipital furrow of the head. In that case the remaining three are the pleural 

 grooves, and there cannot be more than three thoracic segments. It is, however, 

 possible that there may be no occipital furrow, and that the front groove may 

 belong to the thorax. 



There is little chance of confounding M. sculptus with any other British species. 

 The head, however, in its general appearance is a little like the head of M. punctatus. 

 It is, unfortunately, very imperfectly preserved in the only complete specimen yet 

 known ; but the glabella and cheeks appear to differ in no important respect from 

 those of M. punctatus. The principal distinction lies in the margin, which is 

 broader in front and appears to be narrower at the sides than in that species. 

 Moreover, in the former it is ornamented by a row of rather large tubercles ; in 

 the latter it is crenulated throughout, the beads being very much smaller and less 

 elevated than the tubercles of M. sculptus. The tail is the most characteristic 

 portion. In M. lobatus, also, there are well-marked ribs upon the axis and lateral 



