The main stem of the family Eurypteridae is formed by the genus Eurypterus, whose earliest form seems to appear in the Lower Siluric (Frankfort shale) and it persists into the Permic without materially changing. Corresponding to the vigor shown by its longevity is the great preponderance of its individuals and the number of species it has produced. Among the latter very different stages of progress are found. Some have clearly progressed but little, as shown by their terete, undifferentiated abdomen, short blunt telson and short swimming legs. Such forms are E. maria Clarke, E. pygmaeus Woodward and E. minor Laurie. The form and small size of all these suggest an immature condition or an arrested development. This is emphatically true of the peculiar type for which Laurie [1899, p. 588] has proposed the genus Bembycosoma which has a semicircular carapace and a short, conical body, stout, short telson and apparently but nine abdominal segments. This form, represented by a single species, resembles nothing more than our eurypterid larvae from Otisville and it is very possible that these and some other primitive-looking small species of Eurypterus are cases analogous to those occurring among the recent crustaceans where development has been arrested by parasites which prevent molting and result in dwarfed adults retaining larval characters.
One species of Eurypterus, viz, E. kokomoensis [pl. 25, fig. 1] is of special phylogenetic interest as pointing the way which has led from Eurypterus to Dolichopterus, Drepanopterus and Stylonurus. These genera form a group by themselves, well defined by a number of peculiar characters, the most important of which are the great lengthening of the legs, especially of the last pair; the slender body without distinct differentiation of preabdomen and postabdomen and the slender, styliform telson. Connected with the enlargement of the cephalothoracic appendages is that of the carapace which is relatively longer than in all other eurypterids, shows a distinct tendency to become squarish, and develops a broad border. Likewise the greater development of the legs affects the metastoma which in all these genera is very long and subrectangular with deep
