To Sarle's comprehensive statement: "Regarding its [Hughmilleria] resemblance to Eurypterus, it might be said that, but for the marginal position of the eyes and relatively large chelae, this form would easily be mistaken for a species of that genus" [op. cit. p. 1090], we may add from the results of our investigations that the facets of the eyes, the character of the opercular appendage and the cordate metastoma are features indicative of a closer relationship to Pterygotus and that we therefore agree with Sarle that it belongs in one group with the latter genus and Slimonia.
Hughmilleria is a good genus that requires distinction as being of a primitive or generalized character, through which it has points of similarity with a number of other genera. It is certain that, notwithstanding its exterior similarity to Eurypterus, it points, by virtue of its cordate metastoma, the intramarginal to marginal position of the compound eyes, the slightly longer preoral appendages, its slender body, less developed swimming legs and the opercular appendage, to the path of development taken by Pterygotus after separation from its common ancestor with Eurypterus. We consequently find the critical characters of Hughmilleria in the same features as did Sarle and Clarke,[1] notably those evinced in the preoral appendages and marginal eyes, as well as in the other features cited, but with the difference that we take them as denoting a primitive condition. This view is strongly supported by the ontogeny of H. shawangunk, for the great similarity of its nepionic and mature growth stages is another proof that Hughmilleria has progressed less beyond the common ancestor of the eurypterids than either Pterygotus or Eurypterus.
- ↑ The senior author follows Sarle in the preliminary description of the Otisville fauna in seeing in the larger preoral appendages and the marginal eyes the critical differences between Hughmilleria and Eurypterus.
contrast to their respective lengths, are proportionately more robust, and each joint, from the third to the sixth inclusive, carries a pair of ventrally and distally articulated slender, curved spines. It is doubtful if any species of Pterygotus has spines on these appendages; certainly, in several species in which these limbs have been found apparently well preserved, they are lacking.
