4 BRITISH FOSSILS. 



characters of which, at least, imitate best those of trilobites, does not 

 help out the case. The differences pointed out there by Milne Edwards 

 and Audouin reside in the feet and appendages, not in any general 

 shape, prominence of eyes, or sculpture, and we can, therefore, only 

 surmise from general analogy, that certain individuals with the glabella 

 flattened so as to leave the eyes projecting, the body more depressed, 

 and the spines of the front and sides of the head, or the mucro of the 

 tail more developed, may be males ; while greater general convexity, 

 more convex glabella rising nearly as high as the eyes, and a tail not 

 attenuated into the mucro, but simply pointed, may mark the other sex. 

 Fig. 1 we think a female, and in this the eyes seen in front view (Fig. 1 a) 

 do not rise much above the glabella, and the body is convex, the tail 

 simply pointed ; but in others the glabella is depressed, especially be- 

 hind, and the front produced. In Fig. 9, supposed to be a male, the pro- 

 duced angular front is shown. Fig. 8 shows the great length the spines 

 of the head sometimes assume. 



If P. longicaudatus, our var. /3, be identical with this species, and it 

 differs in nothing except the greater development of the processes, the 

 probability of this difference being sexual is confirmed, for there occur 

 in the same slabs individuals exactly alike, some with long spines to 

 head and tail, and others without them. 



Affinities. But one European species is sufficiently like ours to 

 render a close comparison necessary. We refer to the Asaphus mucro- 

 natus (Brongn.), which Professor Burmeister identifies with our var. /3 ; 

 however, he had not seen specimens, and we have them before us. The 

 head of that is regularly convex, the axal and glabellar furrows not 

 deep, the eyes placed further forwards, the margin narrow. The tail 

 is like, but the ribs do not imbricate posteriorly, and the intermediate 

 furrow divides them more equally. The mucro, too, is recurved, as in 

 P. proevus, with which, without being identical, it is closely allied. 

 Asaph. pleuroptyx (Green, cast 18), has 11 or 12 lateral ribs to the tail, 

 extending below the very narrow axis ; but with P. limulurus (Green, 

 cast 16), from the Niagara shale, there are so many points of resem- 

 blance, that we can hardly persuade ourselves they are distinct. Speci- 

 mens in the cabinets of Sir C. Lyell and Dr. Bigsby show that the form 

 is more elongate, the pleurae shorter, the glabella contracted at the base, 

 and the eyes perhaps smaller ; the tail narrow, its lateral ribs faint and 

 but little arched, while the extremity of the axis is more prominent. 

 The head wants the strong marginal furrow, and the posterior spines 

 extend but a little way down (to the fifth rib, Hall). While avoiding 

 the risk of a doubtful synonym, we cannot value these mere proportional 

 characters as specific, and believe that a larger series will unite the 

 above and P. Wetherilli (Green, cast 20), to the English species. 



