shorter than wide. In all eurypterids the preabdominal segments are greatly wider than long and it is hence safe to infer that the carapace also was originally wider than long, and to assume for the prototype a small, short carapace of the width of the preabdomen.
In one section of the eurypterids the lateral eyes are marginal and faceted, and in the other situated on the dorsal surface and smooth. It is important to determine which of the two forms of eyes is the more primitive. Laurie has argued in favor of the eyes of Pterygotus, representing the first group, basing his view on a comparison with the trilobites (on the assumption that the eurypterids are derived from the trilobites), the Scorpionidae and Thelyphonidae, "which must be derived from some way down the eurypterid stem." The Lower Siluric fauna is constituted of species with submarginal eyes and this fact seems to favor Laurie's view.
It is a proper assumption that the appendages of the prototype were undifferentiated. Those of Strabops are unknown with the exception of two segments of the last pair which indicate a primitive form of appendage, and eurypterid larvae have also failed to afford any conclusive evidence bearing on this point. The Siluric genera all exhibit far-reaching differentiation in the legs; this manifests itself especially in the last pair which has been variously transformed either into broad and strong paddles or excessively lengthened; the preoral appendages have been developed into enormous pincers in at least one genus, Pterygotus, but the four pairs of walking legs which lie between these extremes have remained relatively simple in construction, especially so in Pterygotus where all four are alike, rather slender and without spines. Laurie has regarded the simple character of the Pterygotus legs as an argument in favor of the primitive character of the genus. While we do not share this view we agree as to the manifest primitiveness of these four pairs of legs and therefore assign such appendages to the prototype.[1] In Eurypterus the fourth pair has also
- ↑ Laurie has also advanced as a point in favor of the primitive condition of Pterygotus "the apparently much greater development of the epicoxite—a structure common to the eurypterids, Limulus and Scorpion, and therefore probably primitive—in Pterygotus than in the other genera" [op. cit. p. 521]. The evidence in regard to Eurypterus since obtained by Holm shows that the epicoxite is as strongly developed as in Pterygotus.
