Page:The Eurypterida of New York Volume 1.pdf/418

 been different from those of the eurypterids. As a rule the eurypterids show only the attachment areas of the branchiae, but the gills themselves are, as in Limulus, extremely delicate, leaflike appendages visible only under the most favorable conditions, when separated from the body. The gills of Sidneyia as figured lack the thickened attachment scars and are themselves of such size as to overlap several sternites (op. cit. pl. 2, fig. 1; pl. 6, fig. 3), all features that are not found in the eurypterids or Limulus.

The most picturesque feature of the Sidneyia is its fanlike tail, consisting of the last segment and one or more swimmerets on either side. The development of such a tail fin is not entirely absent in the Eurypterida, as evidenced by the bilobed tail of Erettopterus, but it is the telson spine in the latter genus which assumes the finlike form, while here the swimmerets are attached to the anterior side of the last segment. The telson spine is therefore apparently not developed in Sidneyia and it would seem possible that the swimmerets are a further development of the lateral lobes of the last segment. We infer from figures 2 and 3, plate 3 of Mr Walcott's paper that they may even have been articulated. As pointed out by Mr Walcott, this telson is like that of the Macrura and it is a feature foreign to the eurypterids.

It seems to us probable that the Limulava as described are not eurypterids, but constitute a primitive order, though exhibiting some remarkable adaptive features. This order possibly belongs to the Merostomata but it is distinctly allied to the crustaceans in such important characters as the structure of the legs and telson, and is therefore much generalized.

Mr Walcott has rightly pointed out the similarity in the carapaces of Strabops and Sidneyia and suggests that Strabops may have but five pairs of cephalothoracic appendages. Strabops, however, has the eyes and telson spine of an eurypterid and the similarity of the carapaces is probably due to the primitive nature of both genera.