walking legs of Eurypterus, but those of the first pair of somewhat diferent appearance and therefore possibly belonging to a different category of structure. We have also observed this appendage [see text fig. 12; pl. 57, fig. 3].
The coxa of the fifth pair differs in structure and size from those of the preceding legs. It corresponds in its large size to that of the entire leg which surpasses all other limbs in dimensions. It is of rhomboidal form, with a large neck on the anterior inner angle forming the gnathobase. The manducatory edge [pl. 72, fig. 2] is made up of an upper sharp cutting portion and a lower crushing portion consisting of a row of teeth which as a rule become finer posteriorly. The neck often becomes so lengthened as to give the coxa a retortlike appearance, as in Hughmilleria and Pterygotus and especially so in Dolichopterus and Eusarcus, where the necklike extension becomes as long as or longer than the rhomboidal base. This great extension of the gnathobase in the latter two genera is clearly correlated to the great longitudinal extension of the cephalothorax and the forward position of the mouth. In Hughmilleria, Pterygotus and especially Slimonia, the similarity to a retort is still much increased by the rounding outline of the body of the coxa. In Dolichopterus and Eusarcus the immense last coxae cover more than half of the ventral side of the cephalothorax [pl. 44]. Their form is quite characteristic in the different genera and the generic relations of detached coxae are readily recognized. While the large last coxa covers the preceding coxal segment as well as the anterior portion of the first ventral abdominal segment or operculum, the inner portion, except the chewing edge, is in turn covered by the underlip or metastoma.
The mouth, which is situated at about the middle of the ventral side of the carapace, is surrounded not only with the coxae but also by several covering liplike plates. One of these is the epistoma of Pterygotus. This was first described and figured by Huxley and Salter [Monogr. pl. 1, fig. 1]. They were, however, misled, probably by the direction of the sculpture on it, and figured it with the straight margin toward
