them strong evidence of the closer relationship of Hughmilleria to Pterygotus; for it is obvious that the appendage in P. anglicus and P. osiliensis is only a more extreme development of that of Hughmilleria by the expansion of the distal end. As a last character suggesting Pterygotus the small distal expansion of the swimming legs is cited. Considering the wide variation in this character within the genus Eurypterus (compare for instance the swimming legs of Eurypterus remipes and E. kokomoensis), this similarity with Pterygotus is of but little import.
As characters distinctive from Pterygotus, Sarle cites the preoral appendages, the spiniform walking legs and the telson. The difference in the preoral appendages is described as follows:
These are stout, three jointed, chelate organs, so short that when extended they barely equal one half the length of the cephalothoracic shield. The pincers are edentulous and bevel-edged and in their normal position lie folded over the basal joints so that their tips converge close to the anterior border of the mouth. Extended, these organs project beyond the border of the shield for perhaps half their length; when turned straight back, their tips lap over the end of the metastoma. In Pterygotus, on the contrary, these organs are very long, having, at least in Pt. bilobus Salter and Pt. macrophthalmus Hall a length fully one third that of the entire animal; and consist of ponderous, dentate pincers supported on a slender, retrally tapering proximal joint of such a length as must necessarily have prevented the pincers being used at the mouth, unless these appendages were somewhat retractile, as suggested by Laurie.
Though the preoral appendages of Hughmilleria are larger than those of Eurypterus they differ still more widely from the very long appendages of Pterygotus with their dentate pincers, but they may well be considered as indicating a tendency to a development of Pterygotuslike chelicerae.
The difference between Pterygotus and Hughmilleria in the character of the spiniform legs, is clearly set forth by Sarle[1] and the slender lanceolate telson is distinctly a Eurypteruslike feature.
- ↑ The next important difference is in the character of the spiniform walking legs. As in Pterygotus, these consist of seven joints, but the several pairs present a greater
