BRITISH FOSSILS. 



3 



All the middle pleurae have a strong curve backwards from the 

 fulcral point, but at the same time arch outwards, and gradually, 

 as they approach the tail, close in upon it until the hindermost are 

 parallel with it. These lunder pleurse are greatly lengthened, and 

 are of two forms in two distinct varieties (possibly sexes ?). In one 

 form (fig. 4) the penultimate pleura is developed into a shorter spi'ne 

 than the preceding ; and the last is suddenly abbreviated and in- 

 curved. This may be by abortion of the segments. In another 

 the increment is regular, but the last spines are not extravagantly 

 developed. In a third variety, the ultimate and penultimate pleune 

 are greatly extended (fig. 7), and this is accompanied by a corre- 

 sponding dilatation and lengthening of the caudal portion next to 

 be described. 



The tail in this species is most remarkable, and for some time I 

 was inclined to believe that its outer segment was the ultimate 

 pleura, of the body. In fact, the front caudal ring is a very slightly 

 metamorphosed body -joint, and is not very strongly connected with 

 the tail piece ; but it nevertheless belongs to it. 



Exclusive of the great sabre-shaped lateral spines, which are 

 three or four times its length, the tail is an oblong convex plate, 

 with a short conical broad axis occupying about two-thirds of its 

 length, and annulated by two or three incomplete rings. The 

 extremity of this plate is broad and sharply truncate, contrasting 

 with the parabolic contour of its axis, which is not so long as broad. 

 The sword-shaped appendages are broader as well as longer than 

 the last pleura of the body rings, and at first bend strongly inwards 

 beneath the tail, afterwards diverging again at the tips (fig, 4). In 

 one variety they are, in a moderate sized specimen, four inches long. 

 They are connate with the central plate of the tail, though sepa- 

 rated from it by a deep groove, except at the actual base, where the 

 character of a pleura is maintained by the usual pleural groove 

 running out into it. 



The nearest approach to this structure is made by the Para- 

 doxides Bohemicus. But in that species the enlarged last appen- 

 dages are true pleurae, according to Barrande's figure, and the tail 

 itself is destitute of ail appendages. Moreover, in that allied 

 species the second pleura of the body is enlarged ; so we have an 

 additional character of separation from the present species. 



In P. spinosus, Boeck, the glabella is shorter and the eye less 

 curved and nearer the glabella. The unfurrowed portion of the 

 pleurae is shorter, and the hinder pleurae are only straight, not sinuous. 



11 K 2 



