seriatim, bearing in mind that details in structure are in most cases unavailable and that consequently genetic relationships are obscured. Of the genus Stylonurus, three species have been described by Laurie: S. elegans, S. macrophthalmus, and S. ornatus. The first species has been placed by Clarke and Ruedemann into the subgenus Ctenopterus, together with S. cestrotus Clarke, and S. multispinosus Clarke and Ruedemann; the former from the Shawangunk, the latter from the Pittsford, the subgeneric characters being the relatively greater length of the second and third pairs of legs when compared to the first, and the presence on the former of more than two pairs of long, slightly curved spines, which are vertical on the lower side of the segments (Clarke and Ruedemann, 39, 286–287). The Scottish species is so different from the two American forms grouped with it that the author is tempted to take exception to their being placed in the same subgenus, particularly because the very characteristics which are mentioned as diagnostic are not always observable. My reasons for objecting to the subgeneric grouping of this form under Ctenopterus are as follows: (1) It is unsafe to base a taxonomic group of such great value as a subgenus upon the characteristics of one set of organs alone, as for instance, the legs. Nothing at all is known of the body of S. multispinosus and very little about that of S. elegans; only that of S. cestrotus having been found in good enough preservation to allow of restoration. (2) Single identical morphological characters do not of themselves establish specific relationship and, therefore a fortiori they cannot be used to unite their possessors into groups of higher taxonomic value for it is a law of palaeontology which is coming more and more to be recognized, that the same morphological characters crop out in many diverse phyletic groups and their presence in no wise indicates genetic relation. Thus, a modification in the proportions of the legs or in the number of spines cannot be considered characters of subgeneric rank. (3) The length, breadth, general form and grouping of the spines on the second aud third pairs of legs are not at all similar in S. elegans and S. cestrotus (fig. 23). The comparatively short spines of about equal length, regularly spaced, and projecting at almost right angles from the walking legs, in S. excelsior (provided the restoration of this species is correct) and in S. cestrotus, together with the greater length in the second and third pairs of legs as compared with the first pair, might allow of these two species being placed in the same subgenus, and with them quite probably S. multispinosus. S. elegans, however, is too distinct, it
