spines or bristles well adapted to broaden the swimming surface of the limbs. To the first leg of S. excelsior which is the only one observed, paired flat spines are attached whose form suggests their adaptation to swimming. We are disposed to regard all legs as adapted for crawling. Though in some species the forelegs served as swimming organs, the long posterior ones were mainly active in pushing the animal forward.
In another group of species again the forelegs do not differ in relative length and character from the walking legs of Eurypterus. The typical representative of this group is S. logani. This leads us to the discussion of the subdivisions of Stylonurus, distinguishable on the basis of leg structure. On comparing the figures of S. logani and S. macrophthalmus [text fig. 65] with those of S. cestrotus and S. elegans, it is at once evident that in the former the forelegs are not only relatively smaller and narrower than in the other but are also provided with only one pair of spines each, as in Eurypterus, while in the latter group these legs are relatively much longer, more powerful and furnished with a contiguous series of long spines, each segment carrying a greater number of spines than two, a character apparently not repeated in other genera.[1] A third group is represented by S. scoticus Woodward. In this, the single limb known (obviously one of the last pair) is greatly broadened from the beginning and is distinctly a swimming leg. The similarity of this swimming leg to that of Eurypterus would suggest a reference to the latter genus if the other characters of the animal were not so manifestly stylonuroid. Still the species is not a typical Stylonurus but distinctly a later and more specialized form. This is shown not only by the swimming leg but also by the large hooked, winglike epimera of the postabdomen and the sculpture of the tergites. It demands recognition as representing a subdivision of Stylonurus.
- ↑ Laurie's figure of Eurypterus scoticus [1899, pl. 39, fig. 26] shows on the fourth segment three spines, which would suggest a like feature, but since the succeeding segments clearly possess only two spines it would seem that this appearance of a close series of spines is accidental.
