is a unique form, and shows a marked divergence from congeneric forms throughout the world. The telson is elongate, spatulate, with a pronounced median keel, which is represented on the last three segments of the postabdomen as a long spine, rather than a ridge (Woodward, 312, 199, Pl. X, fig. 2; 195, p. 35, Pl. I, fig. 4). There can be little doubt that this species, which is represented by a single, nearly entire individual, represents a neanic stage of some form, the adult of which probably is not known. Only two and a half inches long, it has the large eyes slightly removed from the border, a feature which is so characteristic of neanic Pterygoti; but it is difficult to account for the pronounced spines on the body segments, and for the high keel, features, which in associated species are less developed at so early a stage. The shape of the carapace and the position of the eyes suggest P. macrophthalmus from the Bertie, but the spines on the epimera of the last five segments of the postabdomen, the median spine on the last three, the very marked median keel on the telson as well as the proportions of the telson indicate a specialization far beyond that observable in the species just mentioned, particularly when it is borne in mind that all of these features are observed in an undoubtedly young individual, which means that they would be much more marked in maturity. This species has all of the appearances of an aberrant form, the relations of which it is impossible to determine from the one known specimen, but it certainly has characters which unite it with Bertie species and with formr which occur in the Baltic region.
The Stylonuridæ of the Old Red sandstone are represented by four species: S. scoticus, S. powriei, S. ensiformis, and S. symondsii. The first, represented only by a head and by one nearly entire individual, is yet so remarkable, so entirely distinct from the typical Stylonurus that it has been set apart by Clarke and Ruedemann as the representative of a new subgenus, Tarsopterus. These two authors have dwelt upon what they consider the close similarity between S. scoticus and S. myops from the Shawangunk, stating that "it seems probable, therefore, that S. myops, when fully known, will prove a representative of the subgenus Tarsopterus of which S. scoticus is the type" (39, 303). The reasons which they cite are: occurrence of "spurlike epimera of equal relative size," the "outline of carapace," and "the approximate position of the eyes and the sculpture of the tergites." Since it is my purpose in the present section of this paper to marshal all of the evidence provided by the relationship
