54 BRITISH CAMBRIAN TRILOBITER. 



2. Olenus gibbosus (Wahlenberg). Plato V, figs. 8 — 10. 



1821. Entomostracites gibbosus, Wahlenberg, Petrif. Tell. Suec., p. 39, pi. i, fig. 4. 



1822. Paradoxides gibbosus, Brongniart, Crust. Foss., p. 35, pi. iii, fig. 6. 



1827. Olenus gibbosus, Dalman, Om Palsead., K. Vet. Akad. Handl. (1826), p. 256. 



1837. Olenus gibbosus, Hisinger, Leth. Suec, p. 19, pi. iv, fig. 3. 



1854. Olenus gibbosus, Angelin, Pal. Scand., p. 44, pi. xxv, fig. 5. 



1867. Olenus gibbosus, Belt, Q-eol. Mag., vol. iv, p. 295, pi. xii, fig. 5 a, b. 



Head rather more than twice as broad as it is long, the outline forming a 

 segment of a circle. Glabella prominent, raised above the level of the cheeks, 

 narrower than the cheeks, short, separated from the front margin by a space 

 about equal to half its own length, narrowing slightly towards the front, truncate, 

 with three pairs of oblique glabellar furrows, of which the first pair is very 

 indistinct, occipital furrow well defined. Eyes moderate, crescentic, placed nearly 

 in the middle of the cheeks, distant from the glabella more than half the width of 

 the latter, connected with the anterior angles of the glabella by straight ocular 

 ridges which run nearly at right angles to the axis. Facial suture running 

 backwards and slightly outwards from the anterior margin to the eye and thence 

 more strongly outwards to the posterior margin, which it cuts some distance 

 within the genal angle. The cheeks and frontal limb slope steeply downwards in 

 front of the glabella, less steeply towards the sides ; fixed cheeks rather wide ; 

 free cheeks with faint vascular markings which radiate from the eyes. Margin 

 narrow, upturned in front. Genal angles produced into spines which are directed 

 backwards. 



Thorax of fifteen segments, narrowing backwards from the sixth or seventh 

 segment. Axis narrower than the pleurae. Pleura? straight and nearly hori- 

 zontal, the anterior segments facetted ; anterior segments pointed, the later 

 segments produced into spines ; all the pleurae grooved, the grooves being slightly 

 oblique. 



Tail triangular. Axis narrow, rather less than the width of the lateral lobes in 

 front, almost cylindrical, with either six or seven distinct rings besides the 

 terminal portion, reaching to the posterior margin. Lateral lobes flat, with about 

 four shallow and rather indistinct furrows, the ribs between being interlined. 

 No definite marginal rim ; a minute lateral spine on each side opposite the third 

 axial ring. 



Dimensions. — The British specimens are very variable in size, but sometimes 

 attain a length of 35 mm. or more. 



The presence of this species in the Lingula Flags of the Dolgelly district was 

 recognised by T. Belt. Many of the specimens are complete, but the state of 



