52 BRITISH CAMBRIAN TRILOBITES. 



1. Olenus truncatus (Briinnich). Plate V, figs. 1 — 7. 



1781. Trillin* truncatus, Briinnich, Kong. Dansk. Vidensk. Selsk. Skrift., Nye Samling, Forste Deel, 



p. 391. 

 1827. Trilobites gibbosus var., Boeck (pars), Mag. for Naturv., 1827, p. 24, fig. 8. 

 1838. Trilobites gibbosus var., Boeck (pars), Keilhau's G-sea Norv., p. 143. 

 1843. ? Olenus gibbosus, Burraeister, Organ, d. Trilob., p. 81, pi. iii, fig. 9. 

 1854. Olenus truncatus, Angelin, Pal. Scand., p. 43, pi. xxv, fig. 1. 

 1857. Olcn a* gibbosus var., Kjerulf, G-eol. d. si'idl. Noi-w., p. 284. 



1865. Olenus gibbosus var., Kjerulf, Veiviser ved geol. excursioner i Christiania oraegn, p. 2. 

 1882. ? Olenus truncatus, Brogger, Die Silur. Etagen 2 und 3, p. 98, pi. xii, figs. 5, 5 a— c. 



Our British specimens of this species being very much compressed and dis- 

 torted, the following description is based on specimens from Andrarum in Scania. 



Head about two and a half or three times as broad as it is long, straight in 

 front. Glabella narrow, less than the width of the cheeks, short, separated from 

 the front margin by a space about equal to half its own length, nearly parallel- 

 sided, truncate in front, with three pairs of glabellar furrows and a well-marked 

 occipital furrow. Cheeks wide. Eyes fairly large, crescentic, placed nearly in 

 the middle of the cheeks, distant from the glabella considerably more than half 

 the width of the latter, connected with the anterior corner of the glabella by a 

 straight ocular ridge which runs at right angles to the axis. Facial suture 

 running slightly outwards from the anterior margin to the eye, and behind the eye 

 curving rather strongly outwards and meeting the posterior margin some distance 

 within the genal angle. Free cheeks bent somewhat downwards, with faint 

 vascular markings which radiate from the eye ; the posterior border of the free 

 cheek not quite in line with that of the fixed cheek but turning slightly forwards. 

 Margin narrow. Genal spines short, directed outwards. 



Thorax of thirteen segments, narrowing backwards from about the seventh 

 or eighth segment. Axis a little wider than the pleuras. Pleuras straight, in the 

 first five or six segments bent slightly downwards near the tips and with 

 articulating facets, in the later segments nearly horizontal and not facetted ; in the 

 anterior segments bluntly pointed, in the later segments produced into short 

 spines, the spine on the tenth segment being apparently the longest ; all the 

 pleuras grooved, the groove being rather broad and, except near the extremities, 

 lying in the middle of the pleura. 



Tail small, triangular. Axis broad, consisting of five segments, reaching 

 nearly, if not quite, to the posterior margin ; bluntly conical. Lateral lobes 

 narrower than the axis, with (on each side) three distinct broad grooves, and a 

 fourth indistinct, all the grooves nearly at right angles to the axis, the ribs 

 between the grooves marked by a fine intermediate line. Margin narrow, without 

 spines. 



Dimensions. — Full-grown specimens commonly attain a length of 20 — 25 mm. 



