BRITISH FOSSILS. 



5 



Mr. Gray's cabinet (fig. 8.) has the lenses decidedly small, distant 

 their full diameter from each other, and the intermediate granula- 

 tions more elevated and connected into zigzag lines. Fig. 7 * shows 

 the ordinary surface of the eye. Some specimens have the axis of 

 the body more prominent than others, and the tail is more pointed 

 in some than in others. The glabella varies in width, and divergence 

 of the axal furrows ; many specimens having the sides nearly 

 parallel, as in fig. 4, others, as fig. 10, somewhat more clavate. 

 And in a dwarf variety from the Caradoc sandstone, found by Pro- 

 fessor Sedgwick at Llanrwst, in North Wales, the clavate form is 

 very marked. Occasionally (fig. 4) the two front furrows become 

 quite obscure ; but this is a rare variation. These two upper furrows 

 are always shallower than the lower one and neck furrow, and they 

 show but little in the internal cast ; but they are never quite lost. 

 Fig. 14 is from a fine large head from Ledbury, in Mr. C. Stokes's 

 cabinet ; the glabella furrows are remarkably deep, considering it is 

 an internal cast, and the lobes somewhat more tumid than usual. 



Affinities. — The variation just noticed gives the specimen a great 

 resemblance to a nearly allied species, which, however, belongs to the 

 section Phacops, viz. — P. Stokesii, M. Edwards, (P. macrophthalma, 

 Brongn., t. 1. £ 5., figured in Mem. Geol. Surv., vol. ii. pt. 1. pi. 5. 

 fig. 1). This, which is abundant at Walsall and Dudley, and fre- 

 quently met with in the Wenlock limestones of the Malverns, is easily 

 distinguished from all the varieties of P. Bowningice by the shape 

 of the lowest glabella lobe, which in this is narrow, very strongly 

 marked off from the rest of the glabella by a nearly continuous 

 transverse furrow, and its extremities are terminated by two rather 

 small but strongly marked tubercles, while in P. Bowningice this 

 lobe is always linear and destitute of tubercles. The uppermost 

 glabellar furrow is bent as if broken, while in P. Downingioe it is a 

 simple sigmoid curve. The tail of P. Stokesii has only two or three 

 of the upper furrows of the axis and sides distinct ; P. Bowningice 

 has them all marked, and the side furrows interlined by finer ones. 

 But there is a Lower Silurian species, hereafter noticed, still more 

 nearly resembling ours in all its parts — the P. apiculatus, Salter. 

 In this the general shape of the head, and of the glabella and its 

 lobes, have just the same appearance as those of our species, but a 

 careful comparison will show marks of decided difference in all 

 these parts. In the P. apiculatus, which is as common in the 

 Lower as the P. Bowningice in the Upper Silurian, the head is 

 longer, and the glabella more elongate and narrower, and more con- 



