BRITISH FOSSILS. 



Decade II. Plate IX. 



OLENUS MICRURUS* 



[Genus OLENUS. Dalman (in part). Sub-kingdom Articulata. Class Crustacea. 

 Order Entomostraca. Family Trilobitse.) Glabella narrowed anteriorly, lobed; eye 

 smooth, generally with a narrow prominent ridge connecting it with the upper part of the 

 glabella ; facial suture marginal in front, and cutting the posterior margin behind ; no 

 rostral shield ; hypostome . . . . ? pleurae 14, or fewer, pointed and recurved ; tail with 

 articulated axis and sides.] 



[Sub-genus Olenus. Body rings 14 ; tail entire.] 



Diagnosis. O. ovatus posterius attenuatus ; glabella genisque cequalibus ; 

 thorace segmentis 14, pleuris postremis axi angustioribus ; caudd transversa 

 integrd, apice truncato, axi \-annulato, lateribus unicostatis. 



In the course of surveying the barren country south of Snowdon, Mr. 

 A. Selwyn, of the Geological Survey, discovered fine specimens of this 

 new Olenus. They were found only at two or three localities in the 

 lowest fossiliferous beds of the Silurian system, and may be considered, 

 therefore, as the oldest British trilobites ; they were associated with a 

 species of Eurypterus and a Lingula. The discovery is the more inte- 

 resting, as Olenus and Paradoxides are among the most ancient genera 

 of trilobites in the Silurian rocks of the Continent. Other British 

 species of the group have been described by Mr. John Phillips, from 

 the black shales of Malvern. 



Description. — Length, one inch three-eighths ; width, seven-eighths. 

 General form broadly obovate, acuminate posteriorly ; the axis rather 

 prominent, and the sides flattened. Head more than twice as wide as 

 long, semicircular, with a narrow equal border, and produced at the 

 posterior angles into moderate diverging spines. Glabella reaching 

 very nearly to the front margin, bell-shaped, not very convex, but a 

 little swelled below, with a narrow neck lobe, and two lateral lobes. 

 Cheeks a little wider than the glabella. Eyes not large, forward, near 

 the glabella, and connected with it by a slight oblique prominence. 

 Eye line cutting the posterior margin far outwards. Pleurae 14, flattish, 

 increasing a little in width as far as the 6th segment, then shortened 

 gradually, so that the two last are scarcely wider than the tail ; each 

 marked by an oblique shallow furrow, deeper near the tips, which are 

 slightly recurved and produced into short spines. Axis narrowed pos- 

 [ii. ix.] 2 I 



