466 PYGOCEPHALUS COOPERI 



appendage, and internally lies parallel with the longer cylindrical 

 joints, and in close contiguity with the basal division of the append- 

 age. I believe, in fact, that this filament is nothing less than the 

 outer division of the appendage, or its exopodite ; and I am inclined 

 to think that traces of a corresponding filament are visible in some 

 of the other appendages. 



No. 2 (fig. 3). — This specimen is the more perfect of the two 

 belonging to Mr. Cooper. Like No. 3, it was obtained from the 

 shale overlying the upper or thick coal-beds of Bilston, and is im- 

 bedded in a lighter-coloured ironstone than the foregoing, to which, 

 in other respects, it bears a close resemblance. In fact, it presents 

 precisely the same view of the fossil, and differs from the Manchester 

 specimen chiefly in the absence of the semicircular disk, which is 

 replaced by a deep pit in the relief, with which a corresponding ele- 

 vation in the cast corresponds. The median part of the body and 

 its appendages are similar to those above described, but the traces of 

 the multiarticulate exopodite are but obscurely exhibited : the quad- 

 rate plate is of the same character, but its internal small pair of ap- 

 pendages are hardly traceable, though the external ones (fig. 3, 2") 

 are well preserved. The great feature of interest about the specimen 

 is, that there lies on both sides of the quadrate disk a narrow plate {d), 

 like that found on the one side only of the Manchester specimen. 

 On the left side this plate does not extend further backwards than 

 the proximal edge of the quadrate disk ; but on the right side it is 

 traceable as far as the fourth segment, and has, lying between it and 

 the quadrate disk, the ist, 2nd, 3rd, and 4th appendages of its side. 

 On this side also the plate exhibits a slight transverse furrow ending 

 in a small spine at the point marked (d). Distally, both narrow 

 plates appear to be continuous with the quadrate disk. 



No. 3 (fig. 2). — This specimen is much crushed and distorted, 

 though the segments of the central division of the body, the ap- 

 pendages of one side, and the two scale-like bases of the large 

 appendages of the quadrate plate are clearly visible. The great 

 interest of this specimen proceeds, however, not from the existence 

 of these parts, which are better shown in Nos. i and 2, but from the 

 fact that in the place of the semicircular disk we see two broad 

 plates convex from side to side, and a large portion of a third ; all 

 of them being obviously the lateral moieties of large segments similar 

 to that whose boundary could be traced on the inferior convexity of 

 the semicircular disk in No. i (fig. i /', e). On one side of the 

 specimen (external to b, fig. 2) there lies a plate which probably 

 corresponds with those marked d in the other figures. 



