THE EURYPTERIDA OF NEW YORK I29 
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. | 7 
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 dis- 
regarded 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 prin- 
cipally distinguished by the marginal, faceted eyes; the latter, containing 
the genera Eurypterus, Dolichopterus, Echinognathus(?), Eusarcus, Dre- 
panopterus 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 sepa- 
rated the Pterygotidae from the Eurypteridae. 
First in regard to the Pterygottdae: 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 trilo- 
bites and militate against the probability of the origin suggested. 
