GO BRITISH CAMBRIAN TRILOBITES. 



first segment and runs close to the exterior margin, reaching nearly half way to 

 the genal angle. The thorax seems to consist of six segments and the tail of three, 

 but the division between the two regions is indistinct. The other specimen figured 

 is not so nearly perfect, but shows the margin of the head more clearly. 



The presence of these young forms is, perhaps, an additional argument in 

 favour of the view that Olenus mundus is merely a stage in the development of 

 Olenus cataractes. 



Horizon and Locality. — Lower Lingula Flags : Trefgarn Bridge, Haverfordwest. 



(>. Olenus longispinus (Belt). Plate VI, figs. 6, 7. 



1868. Gonocoryphe ? longispina, Belt, Geol. Mag., vol. v, p. 9, pi. ii, figs. 12 — 14. 



Head nearly semi-circular, with the genal angles produced into long, slender, 

 backwardly-directed spines. Glabella very short, nearly as broad as long, about 

 two thirds the length of the head, and rather less than one third the width, nearly 

 parallel-sided, truncate in front, with two pairs of oblique glabellar furrows. 

 Byes placed in the middle of the cheeks, distant from the glabella rather more 

 than half the width of the latter, united with the anterior angles of the glabella 

 by strong curved ocular ridges which run nearly at right angles to the axis. 

 Facial suture running inwards and backwards from the anterior margin to the 

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

 nearly opposite to the end of the first thoracic pleura. Cheeks moderately convex. 

 Margin narrow ; genal angles produced into long, slender, backwardly-directed 

 spines, which reach nearly as far as the end of the tail. 



Thorax of fourteen segments, widening slightly to the sixth segment, and 

 thence narrowing rather rapidly backwards. Axis about equal in width to the 

 pleurse in the anterior segments, narrower in proportion in the posterior segments, 

 each axial ring with its lateral extremities tuberculate and also bearing a small 

 median tubercle, which, however, is not always very distinct. Pleurse straight, 

 obliquely grooved, the anterior pleurae distinctly bent down at the fulcrum and 

 produced into short outwardly-directed points ; the extremities of the later 

 segments unknown. 



Tail broad and short, rounded in outline. Axis narrower than the lateral 

 lobes, reaching to the posterior margin, consisting of three or four rings. Lateral 

 lobes flat, with three furrows. Margin entire. 



Dimensions. — Length about 18 mm. 



No other British species possesses the very long genal spines characteristic of 

 this species. Olenus truncatus and 0. (jibbosus, both of which have short glabellas, 



