oval preabdomen and narrow taillike postabdomen, anteriorly longest series of endognathites, curved telson and other features, this genus bears marks of a peculiar aberrant type that apparently left no successors.
VI
TAXONOMIC RELATIONS
Any inquiry into the zoological position of the eurypterids must be based on that of Limulus since there is little doubt in the minds of paleontologists of the close relationship between the eurypterids and the king crab. In fact, all recent investigations and discoveries of eurypterids have only served to bring out new homologies of structure between the two groups. Nieszkowski, Hall and Woodward among the earlier writers on eurypterids clearly recognized and distinctly pointed out the numerous homologies and united the eurypterids and xiphosurans. They were especially successful in correlating the segments and the appendages of the cephalothorax. Later Schmidt and Laurie established the fact that the Eurypterida had four pairs of platelike abdominal appendages that bear the branchiae on their posterior surfaces and constitute another important homology with Limulus. Finally Laurie and Holm indicated as a further common structure the existence of preoral appendages in the eurypterids. Holm especially demonstrated this close relationship with the limulids, by the description of a number of finer details of organization: the occurrence of epicoxae in the second to fourth endognathites, the presence of a circular hole spanned by a thin membrane in the coxa of the fourth endognathite. We believe we have added to this array of evidence some further important details in identity of structure of the compound eye of Pterygotus and that of Limulus, and of the general parallelism in the ontogeny of the eurypterids and Limulus. The former argument is considered of especial significance as the eye of Limulus has been shown by Watase and others to be of a type of structure entirely peculiar. In the chapter on ontogeny we have not expressly pointed out the similarities and differences in the individual development of the eurypterids and
