4 



BRITISH FOSSILS. 



(figs. 12, 13), and on the under side of each, in front of this notch, 

 is placed a tubercle (fig. 13, a). When the animal was in the act of 

 rolling up, the tubercle prevented the next ring from being pushed 

 too far forward ; the tail, too, has them on its anterior edge. Some 

 such contrivance as this, for giving compactness to the rolled up 

 form, is probably general in trilobites, and Mr. John Gray, of 

 Dudley, who first drew my attention to it, has succeeded in 

 developing nearly the whole of the under surface of this species. 



The tubercles just mentioned occur on the incurved crustaceous 

 portion (fig. 13, h) of the pleurae, which, in this species is but 

 narrow, while in P. caudatus, Decade II. PL 1., it extends some 

 distance inwards. 



The tail is sub-triangular and rather pointed, nearly twice as 

 wide as long, and moderately convex ; the axis is more convex, but 

 does not rise abruptly from the general surface, nor is it separated 

 from the sides by any distinct axal furrows. It is conical, not so 

 wide as the sides, extending to about four fifths of the length of the 

 tail, with an obtuse scarcely prominent end ; it is crossed by five 

 distinct and two or three obscure rings. The sides have five or six 

 rather deep and curved furrows, which end abruptly at the thickened 

 margin ; smaller and shallower farrows occur between each of the 

 principal ones for the whole length. The incurved under margin is 

 narrow but thick. 



The whole of the upper surface, and the incurved margins of the 

 head and tail, are covered with fine, close, equal granulations ; 

 the hypostome is also equally rough — none of the grains become 

 tubercles, but all remain of equal size. 



Variations. — Among the specimens in the cabinets of Messrs. 

 Fletcher and Gray, occur one or two with the eyes (fig. 3) very 

 considerably larger than usual, so as almost to equal those of 

 P. Stokesii ; the specimens, however, clearly belong to the species 

 we are describing. The following measurements in lines will give 

 an idea of this difference, which is represented in our figure 3 : 



Ordinary specimen : — Large-eyed variety : — 



Length of head - - 5 lines. Length of head - - lines. 



Length of the eye - - 2 „ Length of the eye - 2| „ 



Height of eye - - 1 „ Height of eye - - „ 



The surface, therefore, in one case is nearly double that of the 

 other, and the number of lenses is increased to about 180, the 

 lenses themselves being each a little larger and not distant from 

 one another more than half their diameter. Another specimen, in 



