F. R. Cowper Reed — Undescribed Trilohites. 305 



the tborax was composed of the ten segments wliich we now see 

 flus five segments to fill up the existing gap. 



Axis of thorax gently convex, unornamented, and gradually 

 decreasing in width posteriorly. Pleural portions on each side about 

 one and a half times as wide as the axis, but showing posteriorly 

 a slight increase in relative width. Pleura? straight with parallel 

 margins as far as the distant fulcrum, at which they bend backwards 

 to end in a short broad falcate free point. Each pleura is marked 

 by a strong straight continuous diagonal furrow, curving round at 

 the fulcrum to end at the point. 



Pygidium transverse in shape, broadly semicircular, with a regular 

 rounded margin, and with a distinct marginal furrow marking off 

 a naiTow border. Axis convex, bluntly pointed, very slightly 

 tapering, composed of four rings ; length about two-thirds that of 

 the pygidium, connected with the posterior edge of pygidium by 

 a short, sharply pointed, unsegmented, and less elevated appendix, 

 which interrupts the continuity of the border. On the lateral portions 

 of the pygidium four pairs of pleuree with their diagonal furrows 

 are present, and of these the anterior pair has the furrows the most 

 strongly marked. 



Measurements. 



mm. 



Length of trilobite 37-0 



Length of head-shield 12-0 



Width (estimated) of head-shield 25-0 



Width of middle-shield at base 19-0 



Width of fixed cheek at base G-0 



Length of glabella ... ... ... ... 8-5 



Width of ditto at base 7-0 



Length of pygidium ... ... ... ... 5 • 



Width of ditto 12-5 



Eebiarks. — The subgenus of Olenus to which this species appears 

 to belong is that named ParaboUnella by Brogger.^ The sub- 

 quadrate glabella not reaching the front margin, the wide pre- 

 glabellar jDortiou of the head-shield, the rounded margin, the course 

 of the facial suture, the position and relative size of the eyes, the 

 characters of the occipital ring, the genal spines of the free cheeks, 

 the pygidium with regular bordered edge, not furnished with spines, 

 with a 3-4: jointed axis, and lateral lobes marked with distinct 

 furrows, are the most important features in common between 

 0. Planti and the subgenus ParaboUnella as defined by Brogger. 

 The pi'eseuce of only two instead of three lateral furrows on the 

 glabella is a point of difference, but perhaps not of much importance 

 in this case, as we see on examining Brogger's figure of F. rugosa 

 (loc. cit., tab. iii, f. 3) that an indefinite number of furrows seem 

 to be present in that species, and their irregular character and 

 tendency to unite in the middle also reminds us of 0. Planti. In 

 the case of the pleuree their shape and straight diagonal furrow 

 resemble more closely those of Parabolina spinulosa, but the short 



• Die Silur. Etag., 2 and 3 (1882), p. 102, tab. iii, f. 2a, b, 3, and 4a. 



DECADE IV. VOL. VII. — No. VII. 20 



