18 . BRITISH FOSSILS. 



its convex edge turned forwards (a). The plate is as broad as the 

 head, but not quite half so long, and its longitudinal axis is oblique 

 to that of the body. In the middle line, it presents a flattened area, 

 bounded by a longitudinal ridge on each side, and rounded off in 

 front. Although the structure of this part is somewhat obscure, 

 I have little doubt that it corresponds with the epistoma of 

 P. anglicus, and that it has been thrust out of place and thrown 

 forward. 



To the right-hand side of the carapace, opposite the anterior half 

 -of the eye, two curved, serrated, linear impressions, convex inwards, 

 .appear (fig. la). The anterior and inner of these is not more than 

 half the length of the posterior and outer, and cuts the latter about 

 its middle. I find no corresponding impressions upon the left side. 

 When the impressions are viewed very carefully with a magnifying 

 glass, in a good light, they present minute but important differences 

 The serrations of the short impression are concave and striated, and 

 constricted at their bases. Those of the long impression, on the 

 other hand, are flattened and not constricted, and their series is 

 terminated, posteriorly, by a rounded lobe which forms the posterior 

 angle of a broad process whose boundaries become lost externally. 

 Sufficient, however, remains to show the correspondence between this 

 part and the serrated process of the appendage (c) of P. anglicus, 

 while the teeth of the inner and shorter line are strikingly similar 

 to those which beset the edge of the basal joint of the appendage (&) 

 in that species. 



That the impressions in question are in fact the remains of 

 appendages similar to (6) and (c) becomes a matter of certainty 

 when we examine such specimens as those figured on Plate I. 

 figs. 3, 4, 8, 9. 



In fig. 8. is represented an organ, certainly belonging to P. bilobus, 

 whose basal articulation has as nearly as possible the same form as 

 that of the appendage (c), and presents, along its free edge, the same 

 continuous serration and posterior rounded lobe. The like resem- 

 blance is traceable in the joints of the palp -like appendage, except 

 that in this specimen a structure was observable, not visible in any 

 other, whether of the same or of different species. This is a long taper 

 filament, nearly as long as the ultimate and penultimate joints, 

 lying on their surface, and apparently attached by its thicker end to 

 the distal extremity of the antepenultimate articulation. There are 

 indications that this filament was itself jointed. In figs. 3, 4, 9, 

 the whole appendage, or at any rate its basal joint, is seen in situ, 

 and on comparing the serrated edge of any of these with the outer 



