64 BRITISH CAMBRIAN TRILOBITES. 



The glabella is rectangular, sometimes narrowing slightly towards the front, 

 shorter than in Parabolina, with two or three pairs of glabellar furrows, of which 

 the last two pairs are very oblique ; the frontal limb between the glabella and 

 the anterior margin is fairly wide. The eyes are placed about half way between the 

 anterior and posterior margins and very close to the glabella, with the anterior 

 angles of which they are connected by oblique ocular ridges. The facial suture runs 

 inwards and backwards from the anterior margin to the eye and thence outwards 

 and backwards to the posterior margin. The number of thoracic segments varies, 

 but in the adult (in all the species in which the number is known) it is never less 

 than fourteen. The tail is broad, rounded in outline, marginate, and without 

 spines. 



1. Parabolinella williamsoni (Belt). Plate VI, fig. 12. 



1868. Conocoryphe ? williamsoni i, Belt, Geol. Mag., vol. v, p. 9, pi. ii, figs. 7 — 11. 

 1873. Olenus plantii, Salter, Cat, Camb. Sil. Foss. Mus. Cambridge, p. 11. 



1900. Olenus {Parabolinella) planti, F. R. C. Reed, Geol. Mag. [4], vol. vii, p. 303, pi. xii, 

 fig. 1. 



Head semi-circular. Glabella nearly rectangular, narrowing slightly towards 

 the front and with the anterior angles somewhat rounded, separated from the 

 marginal rim by a broad frontal limb ; two pairs of glabellar furrows, both very 

 oblique and both obsolete near the axial furrows, the last pair meeting in the 

 middle of the glabella, the first pair nearly meeting ; neck-segment defined by a 

 furrow, which becomes weak towards the middle and obsolete near the axial 

 furrows, obscurely divided into three portions by oblique furrows, which pass 

 from the posterior lateral angles of the segment nearly to the middle of the 

 occipital furrow ; the central portion thus defined bears a very indistinct median 

 tubercle. Eyes placed about half way between the anterior and posterior margins, 

 very near to the glabella, with the angles of which they are connected by oblique 

 ocular ridges. Facial suture running from the anterior margin backwards and 

 inwards to the eye and thence backwards and outwards to the posterior margin, 

 which it cuts at a distance from the axial furrow less than the width of the 

 glabella. Margin formed by a narrow raised rim. 



Thorax probably of fifteen segments. Axis as wide as the pleurae in the 

 anterior part of the thorax, but in the posterior part not much more than half the 

 width of the pleurae, some of the rings showing very obscure indications of median 

 tubercles. Pleurae straight, obliquely grooved, slightly bent down at the fulcrum, 

 which in the anterior pleurae is placed very near the axis, in the posterior pleurae 

 far out ; the anterior pleurae distinctly facetted and terminating in points directed 



