410 NEW YORK STATE MUSEUM 
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. czt. 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, con- 
sisting of the last segment and one or more swimmerets on either side. 
The development of such a tail fin 1s not entirely absent in the Euryp- 
terida, 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 Merosto- 
mata but it is distinctly allied to the crustaceans in such important char- 
acters as the structure of the legs and telson, and is therefore much gen- 
eralized. 
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. 
