2 



BKITISH FOSSILS. 



(17 to 20) segments to the body, easily distinguish this group, 

 which comprehends the largest Trilobites known, and yet is the 

 earliest or nearly the earliest type of the whole Trilobite family. 

 Agnostus accompanies it in all countries where it has been observed, 

 and it is known to have ranged from N. America to Eussia, and 

 from Sweden to Spain and Bohemia. Twenty years ago a single 

 specimen was found in the British slate rocks, and it is only within 

 the last year or two that it has been found in any considerable 

 numbers, in a single locality in South Wales. I had myself the good 

 fortune to discover the new species, which was first figured in the 

 Quarterly Geological Journal for 1863. 



Description. — Of the head we have now many specimens, and 

 some of the fragments betoken a fossil not less than 16 or 18 inches 

 long ; one or two heads are perfect, and show that it v/as semi- 

 circular, with very large, thick, cylindrical, and tolerably straight 

 spines. The glabella rather long, reaching and overhanging the 

 front margin, broader but not suddenly so in front, half its length 

 being occupied by the great front lobe. 



There are obscure traces in some specimens of short anterior fur- 

 rows, but I cannot be sure of more than the two complete posterior 

 ones, which bend backwards in the middle, and are equally strong 

 with the neck furrow. The eye is far forward, in advance even of 

 the second or upper glabella-furrow, and is near the glabella, — not 

 half its length distant from it. The cheek is coarsely granular, 

 except toward the outer angle, and abruptly contracted beneath, at 

 the base of the great C3dindrical spine. 



The labrum is expanded at the base, and has a truncated end, 

 with sub-spinous lateral angles. It is, as usual, separated by 

 scarcely any suture from the hypostome, or rather is connate with it. 



I can, in a fine specimen lately found, count 19 body-rings, and 

 believe this to be the full number. The axis is ver}^ wide (in the 

 largest specimen 1^ inch) and convex, fully as wide in front as the 

 pleurae, spine included, and so for the eight or nine front segments. 

 The apex of the pleurae in these is abruptly turned back, with a short 

 sharp mucro, and there is no enlargement of the second or third 

 pleura — s, character of importance in this genus. All have a deep 

 groove, which is considerably oblique, and reaches the hinder margin 

 just at the base of the spine in all the pleurae. But from the eighth 

 or ninth segment the pleurae lengthen, and the axis graduall}^ tapers. 

 The hindermost axial ring is about half the width of the front ones, 

 and scarcely one- fourth as wide as its long pointed pleurae. 



