BRITISH FOSSILS. 



s 



The line of the fulcral points is parallel to the axis for all its 

 length, and the constriction beneath them, though not very marked 

 on the upper crust (fig. 8), produces a longitudinal ridge on the 

 under surface, and a strong furrow in casts (fig. 9). Tail, at least in 

 Dudley specimens, very much narrower than the body, with three 

 strong spinous lateral lobes on each side directed backwards, the 

 outer ones a little divergent and longest ; all extend equally back- 

 wards, — the tail is therefore truncate — but exclusive of the spines, 

 it is broad triangular, following somewhat the shape of the axis ; it 

 is marked on each side by four short deep puncta or furrows, which 

 do not run to the margin in young individuals. The axis is convex 

 and short conical, of three distinct ribs and a small terminal piece 

 — the last very obscurely indicated ; there is no mucro between the 

 lowest spines in the ordinary Wenlock forms. 



Variations. — The following have been observed. In a Dudley 

 specimen the front or forehead lobe occupies much more than half 

 the length of the glabella, the side lobes being therefore more 

 crowded. In a Dudley specimen, a large tubercle occurs in the 

 middle of the forehead lobe. In some individuals the glabella 

 widens more above, in others it is nearly parallel-sided, and the 

 lateral furrows vary in leng-th. The head spines occasionally reach 

 the third thorax segment. The margin of the cheek in one specimen 

 is notched at the facial suture (fig. 10, a). The axis of the thorax is, 

 sometimes, though rarely, as wide as the pleurse. The most im- 

 portant variations occur in the tail, — in fig. 5, we have represented 

 the spines as all directed backwards, and the two central ones closely 

 approximate ; they are so in the large Ledbury specimen figured in 

 the " Silurian System,''' where too they are shorter than the outer 

 spines. In fig. 6, they are a little space apart ; in a Lower Silurian 

 specimen we have seen a small tubercle appear between, and in our 

 var. /3 a decided, though short, mucro protrudes. Lastly, as a mon- 

 strous variety fi:om the Silurian rocks of Kildare, — we have reason 

 to think it of the same species, — we have one with a wider interval, 

 and a bifid mucro. In old specimens, as well as in var. /3, the spines 

 diverge much more than in those we have here figured. Perhaps 

 some of these variations are due to sex. 



Affinities. — The considerable variations above mentioned lead us 

 to believe that the Gh. insignis, Beyrich, may be but a variety of 

 this species. We have not materials enough to justify our recording 

 it as a variety, as Beyrich describes and figures it as with a much 

 wider glabella, the furrows reaching but a short way across. The 

 hypostome is very similar, and the tail difiers very little, except in 

 [vii. ii.] 7 B 3 



