and will now proceed to trace the development of the different branches of eurypterids upward from this prototype as expressed in the appended genealogical tree.
Strabops is such a generalized type that it is eminently fitted to serve as prototype; since it is the earliest eurypterid known it may be an actual progenitor of most Siluric forms. Its relatively great width will be disregarded here as not characteristic of the prototype. It suggests that Strabops is already nearer to the Eurypterus than to the Pterygotus stock.
The Siluric and later genera of the eurypterids distinctly fall into two groups or stocks, namely, that of Pterygotus and that of Eurypterus. The former group (Hughmilleria, Slimonia, Pterygotus, Erettopterus) is principally distinguished by the marginal, faceted eyes; the latter, containing the genera Eurypterus, Dolichopterus, Echinognathus(?), Eusarcus, Drepanopterus and Stylonurus, by the smooth, intramarginal or dorsal eyes. There are many differences in the two which indicate that they separated early and developed as independent stocks. We have, therefore, here separated the Pterygotidae from the Eurypteridae.
First in regard to the Pterygotidae: We have elsewhere analyzed the genetic relationship of Hughmilleria to Pterygotus and Slimonia as shown by the position and character of the lateral eyes, the larger chelicerae, the genital appendage and the telson, and have intimated that in the characters of these organs Hughmilleria clearly evinces a more primitive structure than the other two genera. This view is further supported by the more slender form of the abdomen and the shape of the carapace; the latter while more elongate than that of Pterygotus exhibits a distinct
extremely interesting but we have thus far not become aware of facts, in either the ontogeny of the eurypterids or of Limulus and the scorpions, that would suggest such mode of origin of the eurypterid telson spine; indeed the absence of median spines on the dorsal segments of any eurypterids as well as the very early fixation of the number of segments in the Eurypterida, contrast to the variability of these features in the trilobites and militate against the probability of the origin suggested.
