58 BRITISH CAMBRIAN TRILOBITES. 



which is found along with it at Trefgarn Bridge. So close, indeed, is the 

 resemblance that the latter may ultimately prove to be only a young stage of 

 0. cataractes. There are, however, certain differences which appear to be 

 constant, and I have not yet discovered any intermediate forms. In 0. mundus 

 the ocular ridge is much more strongly marked, the pleural spines are shorter, 

 the axis of the thorax bears a median tubercle upon each segment, and the axis 

 of the tail has only two or possibly three rings differentiated upon it instead of 

 four. Owing, however, to the small size of the specimens and the increasing 

 vagueness of the segmentation towards the end of the axis, but little importance 

 can be attached to the last character. 0. cataractes is usually considerably larger 

 than 0. mundus, but occasional specimens approach the latter in size. 



Salter's figure of Olenus cataractes is evidently a restoration, but he states that 

 the specimen is from Treflys and is in the Museum of Practical Geology, and 

 hence it would appear that the specimen upon which the restoration is chiefly 

 based must be that which is shown in Plate V, fig. 13. He lays some stress upon 

 the fact that the second and third pairs of glabellar furrows are continuous across, 

 but this is a peculiarity which is often met with in other species of the genus and 

 which appears to depend upon the mode of preservation — being due, in fact, 

 merely to the crumpling of the test between two points of weakness. The really 

 continuous glabellar furrows in such forms as Sphd&rophthalmus alatus present a 

 very different appearance. 



Owing to the imperfection of his material, Salter failed to observe the lateral 

 spines of the tail, but they are very clearly shown in the beautiful specimens 

 collected by Mr. Turnbull at Trefgarn Bridge, and they are distinctly visible in 

 some of those from the Maentwrog Falls, although, owing to the compression and 

 distortion of the specimens, they are liable to be overlooked. 



Horizon and Localities. — Lower Lingula Flags : Caen-y-coed, Maentwrog 

 Valley ; Treflys, Criccieth ; Portmadoc ; Tal-y-sarnau ; Trefgarn Bridge, Haver- 

 fordwest. 



5. Olenus mundus, sp. nov. Plate VI, figs. 2 — 5. 



General form depressed, ovate. 



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

 Glabella not quite so wide as the cheeks, narrowing very slightly forwards, 

 truncate in front, reaching nearly to the anterior margin, from which it is 

 separated by a space rather wider than the margin itself ; neck-furrow strong ; 

 three pairs of glabellar furrows, of which the second and third are strongly 

 marked and very oblique, while the first is fainter and less oblique. Cheeks 

 rather wide ; free cheeks with faint vascular markings radiating from the eye. 



