b Postoral appendages. Of the five pairs of postoral limbs which are frequently designated as endognaths or endognathites, the first and fifth show the greatest amount of differentiation, while the intervening three pairs, as a rule, are very much alike and are functionally uniform, mostly serving as walking legs. We find the same condition in both Limulus and the scorpions and may therefore infer that the intermediate legs retain the original condition and that differentiation most easily affected the most exposed pairs, the first and last.
Generally all legs increase in length regularly from in front backward. This condition is typically shown in the more primitive genera Drepanopterus, Eurypterus and Hughmilleria.
The legs of Drepanopterus [pl. 54] exhibit the least differentiation of all genera, whose legs are known. All five pairs form a series of limbs which increase in length backward and are very much alike. One distinction, however, is that the first three are provided with a pair of fairly large spines on each segment, the posterior being longer than the anterior, an arrangement which was obviously of great assistance in pushing the body forward. The spines become gradually reduced backward in the series of legs; in the third pair posterior spines only are still well seen, the anterior ones being reduced to mucros. On the last pair they are all reduced to mucros. From the legs of Drepanopterus those of Stylonurus can be directly derived.
In Hughmilleria the first four pairs form continuous series of walking legs, all four being equally spiniferous and undifferentiated.
In Eurypterus the structure of the legs has been most minutely described by Schmidt and Holm in E. fischeri, and our large collection of E. remipes and lacustris corroborates their excellent work. The first three pairs are thick and heavy and increase in length regularly backward. The first leg consists of seven, the second and third of eight segments each, the terminal claw included. They are convex on the upper and flat on the underside and so articulated that they can be bent only downward. The principal spines are articulated and paired, the
