104 BRITISH FOSSILS. 



Plate XIII. fig. 18. There is one other fragment in the "Upper 

 Ludlow to which a place cannot be yet assigned, and yet belongs 

 apparently to P. punctatus. It is a long strap-shaped piece, two 

 inches and a half long and half an inch broad, and narrowed and 

 rounded at one if not both ends. At 6 it is imperfect. The sur- 

 face shows the peculiar long tubercular plicae of the species, at least 

 near the base (a) and the opposite end b, and the margins are 

 deeply incised by short slits forming nearly square serrations (like 

 those on the edge of the carapace of a crab). This is not exactly 

 the structure of any part of the margin of Pterygotus, the usual 

 ornament being that of prominent superficial plicae or tubercles. 

 The terminal (?) portion b differs in not having (so far as the impres- 

 sion shows) this serrate edge, but only a thickened crenate margin, 

 and is probably a distinct joint, or may even be accidentally placed 

 where it is. The surface shows elongate plicse of various sizes, 

 rather thickly scattered. 



The Metastoma (Tlate XL fig. 4) is nearly three times as wide 

 as long, widest above the middle, and tapering backwards with 

 straight sides from this point to the blunt posterior extremity ; 

 the anterior end is broadest, rounded, and not even emarginate ; 

 much less bilobed as in other species. It is thus quite different in 

 shape from that of any other species ; its long oval form, blunt 

 ends, and straightened sides enable us to recognize it at once. It 

 occurs three inches and a half long ; and in specimens of this size, 

 the width is one inch and a quarter. 



The Swi/mmiTig Feet (Plate XL figs. 12-15) are very different in 

 proportion to those ofP.anglicus, the terminal joints occupying a con- 

 siderably greater length, and being abruptly wider than the rest. Of 

 the great basal joint (co in fig. 5) but little is preserved, but a larger 

 specimen, fig. 12, shows it to have been roughly squamose, especially 

 along its basal edge, the narrow squamae projecting as small spines, 

 the rest of the surface is closely imbricated with smaller plicse. 

 PI. XIII. fig. 14, is very possibly the serrate inner lobe of this joint. 

 The second joint b is large in proportion to the rest, and widens from 

 the base to its truncated apex. The third i is subtriangular, the blunt 

 apex of the triangle being anterior, and the edge articulating with the 

 next joint nearly straight or but slightly curved. The fourth joint, 

 on the contrary, is an obtuse triangle, of which the broad base is 

 forward and moderately arched, but not projecting as in P. anglicus. 

 It contracts rapidly behind, where it has a narrow deep notch to 

 receive the articulating process of the next joint. This, ca, is the 

 fifth, a remarkably short wide articulation, almost buried in the 



