68 BRITISH CAMBRIAN TRILOBITES. 



but traces of such a tubercle are present, as distinct as they usually are in so 

 crushed a specimen. 



None of the other species of Parabolinella known in Britain have four pairs of 

 glabellar furrows. 



Horizon ami Localities. — Tremadoc Slates : Nant-y-Glas-dwr and Allt Pen-y- 

 Coed dingle, near Carmarthen. 



4. Parabolinella triarthra (Callaway). Plate VII, figs. 4 — 12. 



1877. Olenus triarthrus, Callaway, Quart. Journ. G-eol. Soc, vol. xxxiii, p. 666, pi. xxiv, fig. 6. 



Head semi-circular, somewhat angulate in front, with the genal angles 

 produced into long spines. Glabella almost square, a little wider in front than 

 behind ; anterior border slightly emarginate, separated from the marginal rim by a 

 broad frontal limb ; three pairs of glabellar furrows ; the first pair often obsolete 

 in small specimens, in larger specimens consisting of (1) a very short deeply 

 impressed groove opening into the axial furrow and directed obliquely forwards 

 and inwards, (2) a much shallower and disconnected furrow nearer the central line 

 of the glabella and slightly further back ; the second pair of glabellar furrows 

 somewhat sinuous or irregular in form, directed obliquely backwards, not quite 

 reaching either the axial furrow or the middle line of the glabella ; the third pair 

 somewhat similar but towards the axial furrow bifurcating, the posterior branch 

 being but faintly impressed; neck-segment well defined throughout, sometimes 

 divided as in P. mgosa, and bearing a median tubercle. Eyes small, placed 

 nearer to the anterior than to the posterior margin, very close to the glabella, a 

 short ocular ridge running from the eye towards the anterior angles of the glabella. 

 Facial suture running from the anterior margin backwards and very slightly 

 inwards to the eye, and thence outwards and backwards almost in a straight line 

 to the posterior margin, which it cuts at a distance from the axial furrow rather 

 less than the width of the glabella Frontal limb broad, about one third the 

 length of the glabella; fixed cheeks narrow in front of the eye, widening rapidly 

 backwards, with a strong neck-furrow ; free cheeks of moderate width, marginate, 

 sloping downwards from the eye. Head surrounded by a narrow marginal rim 

 and furrow, the furrow in front of the cranidium being marked by a row of small 

 pits ; genal angles produced into long spines which extend backwards nearly two- 

 thirds the length of the thorax. 



Thorax with a variable number of segments, but in the larger specimens 

 usually twenty-one, of which the last is more or less firmly attached to the tail. 

 Axis tapering slowly and regularly from front to back so that the axial furrows are 

 nearly straight ; forming rather less than one-third the total width, being relatively 

 narrowest about the middle of the body ; each segment with a median tubercle, 



