offer many new illustrations of this law which is so simple, which so strongly makes its appeal to the reason, and which yet is so constantly ignored. The Carbonic species of Eurypterus develop spines wherever possible; the surface scales are produced into pointed wedges or spines; the ends of the epimera grow out to a great length; spines develop on the appendages not only in rows along the various segments but also on the lines of junction between segments: showing that the final expression of morphological characters in the eurypterids was the development of spines which was followed by extinction. Such a development has seemed expectable to many authors for the species living in the late Palæozoic, in the Mississippic, and Carbonic; but there is really nothing to prevent these phylogerontic characters from appearing much earlier. And so, to apply all of these general statements to the case in question, I would say that the epimeral spines observable on S. myops indicate that the line which that species represented was on the decline even in the Siluric, at a time when the majority of eurypterids were at their acme. A glance at the illustration of S. scoticus (Woodward 312, Pl. XXII) will show to the reader that this species has a typically gerontic appearance. Its epimeral prolongations do not in the least resemble those in S. myops, but are most like those of S. macrophthalmus from the Ludlow.
Two points remain as supposedly indicative of relation between these two species. The position of the eyes is, it seems to the author, the only feature of marked similarity, but certain of the British forms also show such a position, so that it is not of striking importance. As for the ornamentation of the tergites, I can see little to warrant the statement that the sculpture is similar in the two species.
The species Stylonurus (Tarsopterus) scoticus has now been compared in detail with S. myops and it has been shown that they are not closely related and consequently the presence of the first genus in the Old Red sandstone not only does not militate against my thesis that the faunas living in rivers coming from the same continent and in the same latitude should be most alike, but it is actually an additional proof, for S. scoticus is most nearly related to Ludlow and Old Red species, though it shows phylogerontic characteristics which somewhat obscure its relations.
The three remaining species of Stylonurus from the Old Red may be quickly dismissed. S. symondsii, from England, is represented by a single apparently complete carapace which is almost as long as wide, but is distinctly narrower posteriorly than anteriorly. There
