The absence of antennules at any stage in development allies _Limulus_ so closely with the Arachnida and separates it so far from the Trilobita that in recent years there has been a tendency to give up the attempt to prove a relationship between the merostomes and trilobites, especially since Clarke and Ruedemann, in their extensive study of the Eurypterida, found nothing to indicate the crustacean nature of that group. A new point of view is, however, presented by the curious _Sidneyia inexpectans_ and _Emeraldella brocki_ described by Walcott from the Middle Cambrian. =Sidneyia inexpectans= Walcott. Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1911, p. 21, pl. 2, fig. 1 (not figs. 2, 3); pls. 3-5; pl. 6, fig. 3; pl. 7, fig. 1. The body of this animal is elongate, somewhat eurypterid-like, but with a broad telson supplied with lateral swimmerets. The cephalon is short, with lateral compound eyes. The trunk consists of eleven segments, the anterior nine of which are conspicuously wider than the two behind them, and the telson consists of a single elongate plate. On the ventral side of the head there is a large hypostoma and five, pairs of appendages. The first pair are multisegmented antennules. The second pair have not been adequately described. The third are large, complex claws, and the fourth and fifth suggest broad, stocky endopodites. Broad gnathobases are attached to the coxopodites of the third to fifth pairs of appendages and form very strong jaws. The first nine segments of the thorax have one pair each of broad filiform branchial appendages, suggestive of the exopodites of trilobites, but no endopodites have been seen. The tenth and eleventh segments seem to lack appendages entirely. =Emeraldella brocki= Walcott. Illustrated: _Sidneyia inexpectans_ Walcott _partim_, Smithson. Misc. Coll., vol. 57, 1911, pl. 2, figs. 2, 3 (not fig. 1);--Ibid., 1912, p. 206, text fig. 10. _Emeraldella brocki_ Walcott, Ibid., 1912, p. 203, pl. 30, fig. 2; text fig. 8;--Ibid., vol. 67, 1918, p. 118 (correction). _Emeraldella_ has much the same shape as _Sidneyia_ and the same number of segments, but instead of a broad flat telson, it has a long _Limulus_-like spine. The cephalon is about as wide as long, and eyes have not yet been seen. The body consists of eleven segments and a telson (Walcott says twelve and a telson but shows only eleven in the figures). Nine of the segments, as in _Sidneyia_, are broad, the next two narrow.