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.
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.