56 BRITISH CAMBRIAN TRILOBITES. 



near the anterior border. Margin forming a raised rim, produced into a very short 

 spine on each side at the anterior angle. 



Dimensions. — The length of the type-specimen shown in Plate V, fig. 12 must 

 have been about 3-~) mm. 



The length of the glabella, which reaches nearly to the anterior margin, 

 distinguishes Olenus micrurus from all the other British species of the genus 

 excepting 0. cataractes and 0. mundus, and from these forms it is separated chiefly 

 by the characters of the thorax and the tail. In 0. cataractes and 0. mundus the 

 general shape is ovate, while in 0. micrurus the body tapers rapidly from the 

 sixth or seventh thoracic segment, and the pleurae of the last tAvo segments are 

 very short. The tail of 0. micrurus is quite characteristic, with only one ring 

 differentiated upon the axis, and with the lateral lobes narrow and showing only 

 one faint furrow near to the anterior border. 



The presence of the small spine at the anterior angles of the tail is not noted by 

 Salter, but it is distinctly visible in the specimen figured on Plate V, fig. 12, which 

 was one of his types. It is, however, by no means conspicuous, and can only be 

 seen when the specimen is lighted in a particular direction. 



Olenus micrurus is commonly quoted as a characteristic species of the Lingula 

 Flags, but so far as I am aware the only specimens in our museums which can be 

 referred with certainty to this species, are the two from near Trawsfynydd which 

 are shown in Plate V and upon which Salter's figure appears to have been based. 

 The specimens from the Cwm-y-Swm Mine and from Marchllyn Mawr, to which he 

 refers, are much too imperfect for specific determination. 



Horizon <iu<i Locality. — Lower Lingula Flags: Trawsfynydd. 



4. Olenus cataractes, Salter. Plate V, figs. 13 — 17; Plate VI, fig. 1. 



1864. Olenus cataractes, Salter, Mem. G-eol. Surv., Brit. Org. Rem., dec. xi, pt. viii, p. 1, pi. viii, figs. 



14, 14 a, b. 

 1866. Olenus cataractes, Salter, G-eology of North Wales, Mem. Geol. Surv., vol. iii, p. 300, pi. v, figs. 



23, 23 a, b. 

 1878. Olenus caractaci, Salter MS., Cat, Camb. and Sil. Foss. Mus. Pract. Geol., p. 10. 



Head semi-circular, marginate, produced into spines at the genal angles. 

 Glabella about one third the width of the head, reaching nearly to the anterior 

 margin, nearly parallel-sided but narrowing very slightly forwards, rounded in 

 front, with a well-defined neck-furrow and three pairs of oblique glabellar furrows, 

 of which the first is very faint. Cheeks wide, free cheeks sometimes with faint 

 vascular markings radiating from the eye. Byes small, crescentic, placed a little 

 in front of the middle of the cheeks, and distant from the glabella about half the 



