PYGOCEPHALUS COOPERI 465 



On the left side the appendage has a similar structure, but is less 

 perfectly preserved. 



The small appendages (/') of the quadrate disk lie between the 

 large ones. They consist of two proximal subcylindrical short joints, 

 which do not equal more than half the length of the scale of the 

 outer appendage ; beyond these, traces are visible of another joints 

 and of a long, multiarticulate, terminal filament. 



On the one side of the specimen, the matrix comes close. up to 

 the edge of the quadrate disk ; but on the other, an elongated, nar- 

 row plate {d), with a somewhat excavated anterior margin, joins it 

 at the base of the scale-like appendage. This plate is exposed for 

 about -jg-nds of an inch in width and fths of an inch in length. Its 

 outer margin is straight, and slopes outwards so that proximally it is 

 Y^-ths of an inch distant from the axis of the specimen, while distally 

 (or at the base of the scale) it is only \ inch distant from the same 

 imaginary line. A narrow triangular space, occupied by matrix, 

 is left by the divergence of the plate from the central part of the body, 

 and lodges three of its appendages on this side. 



The central part of the body (b) measures about half an inch in 

 length ; it is narrowest towards the quadrate disk, widest at the 

 opposite extremity, where it attains nearly xV^hs of an inch, and 

 is divided into seven segments of nearly equal length, but gradually 

 increasing breadth. Each segment appears to consist of a median 

 plate, separated by an oblique furrow from two lateral plates ; the 

 latter are quadrate, with their margins concave outwardly and to- 

 wards the quadrate disk ; convex and somewhat raised, towards the 

 semicircular disk. The median plate increases in width from the 

 segment nearest the quadrate disk, which we may call the first, to^ 

 the last. The lateral plates are nearly of the same size throughout,, 

 except the first pair, which appear to be larger than the others. 



Attached to the outer boundaries of the lateral plates, seven ap- 

 pendages are observable on the left side, but only six on the right. 

 In the more perfectly preserved appendages (fig. i c) there may be 

 distinguished a short, proximal, convex, subcylindrical joint, followed 

 by at least three other slender and delicate articulations ; of these 

 the proximal one is the longest, the terminal next in length, and the 

 middle one shortest. 



The terminal joint exhibits indications of further subdivision. 



The fifth limb on the right side (the left side of the animal, as we 

 shall see) presents a very important character, inasmuch as there lies 

 parallel with and behind it a delicate, curved, many-jointed filament 

 (fig. I c), which externally abuts against the terminal joint of the 



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