THE EURYPTERIDA OF NEW YORK | 53 
were especially active in aiding the movement of the following seventh 
and eighth segments which form the oar blade. The seventh segment 
is a flat trapezoidal plate possessing on the posterior side a triangular 
lobe, that served as a guard to the eighth. The latter is oval and has a 
terminal notch, in which a minute claw is inserted that represents a ninth 
segment and was termed the palette by Hall, while later authors have more 
properly applied the term palette to the entire eighth segment. 
The species which seem to us of especial interest in the explanation of 
the strange structure of the swimming leg are Eurypterus (Onych- 
opterus) kokomoensis and Dolichopterus macro- 
chirus. . The former [pl. 26, fig. 2] ‘shows the most primitive 
shape of the swimming leg known to us. This primitive character manifests 
itself most distinctly in the following features. Beginning at the distal 
end, the ninth segment is a well developed terminal claw, such as is found 
on the other legs, indicating that the minute ninth segment of Eurypterus 
is a reduced terminal claw. The seventh and eighth segments do not yet so 
exactly fit together into a single oar blade, the seventh being still narrower 
and the eighth more expanded. The triangular guard lobe of the seventh 
segment in Eurypterus is here represented by a long relatively narrow 
lobe, indicating that it originated from a broadened and flattened spine of 
the seventh segment. A glance at figure 26, plate 2, will show that the 
preceding joints also are still much more uniform in character and like 
those of the walking legs in Eurypterus. In Dolichopterus 
macrochirus [see restoration, pl. 40] on the other hand, the ninth 
segment has been developed into a third element of the oar blade which there- 
by has become still more powerful. Here we find triangular guard plates on 
both the preceding, the seventh and eighth segments, which by their form 
still distinctly indicate their origin from spines. The lobelike projecting 
anterior portions of the distal edges of the seventh and eighth segments 
also suggest a similar origin, especially if we compare them with the lobe- 
like spines of the fourth pair of postoral limbs, already noted. 
The postoral limbs performed a still further function, viz, mastica- 
