OLENUS MICRURUS. 55 



preservation is poor. It is, however, readily distinguished from all the other 

 British species of the genus except 0. truncatus and 0. long is pin us by the shortness 

 of its glabella. From 0. truncatus it is distinguished by the rounded outline of the 

 head, the backwardly-directed genal spines, the narrow axis of the tail and thorax, 

 and the larger number of rings on the axis of the tail. The thorax consists of 

 fifteen segments instead of thirteen as in that species, and the tail bears minute 

 lateral spines. Owing to the state of preservation, however, it is seldom possible 

 to make use of the last two characters. 



From O.longispinus it is distinguished by the course of the facial suture in front 

 of the eye, the number of segments in the thorax and the tail, and the outline of 

 the tail, which, in 0. longlspinus, is circular rather than triangular. 



Horizon and Locality. — Lower Liugula Flags : Tyn-y-groes, Dolgelly. 



3. Olenus micrurus, Salter. Plate V, figs. 11, 12. 



1849. Olenus micrurus, Salter, Mem. Greol. Surv., Brit. Org. Bemaius, dec. ii, pt. ix, p. 1, pi. ix, 



figs. 1, 2 (fig. 3 doubtful). 

 1866. Olenus micrurus, Salter, Geology of North Wales, Mem. Greol. Surv., vol. iii, p. 300, pi. ii, 



fig. 5 (fig. 6 doubtful). 



General form depressed, narrowing somewhat rapidly towards the tail. 



Head semi-circular. Glabella forming about one third the width of the head, 

 reaching forwards nearly to the anterior margin ; almost oblong in shape, but 

 narrowing very slightly and somewhat rounded in front, with a well-marked 

 neck-furrow and two (or possibly three) pairs of oblique glabellar furrows. Eyes' 

 moderate, set a little in front of the middle of the cheeks and distant from the 

 glabella less than half the width of the latter, united with the anterior extremity 

 of the glabella by an oblique ocular ridge (which is very imperfectly shown in the 

 type specimen). Facial suture running from the anterior margin backwards to the 

 eye, and thence backwards and outwards to the posterior margin, which it cuts some 

 distance within the genal angle. Margin narrow, even in width. Genal spines 

 short, directed somewhat outwards as well as backwards. 



Thorax of fourteen segments, increasing slightly in Avidth to the sixth or 

 seventh segment, and thence decreasing rapidly, the last two segments being very 

 narrow. Axis forming nearly one third of the total width in the anterior segments, 

 more than one third in the posterior segments. Pleura? straight and horizontal, 

 obliquely grooved, the anterior pleuraa slightly facetted ; all, except perhaps the 

 first two, produced into backwardly-directed spines, which appear to attain their 

 greatest length about the seventh segment. 



Tail very small, subquadrate, slightly emarginate behind the axis. Axis 

 cylindrical, wider than the lateral lobes, reaching to the posterior margin, showing 

 only one ring marked off from the rest. Lateral lobes flat, with a faint furrow 



