The ventral side of the cephalon has a long hypostoma, and five pairs of appendages. The first pair are very long multi segmented antennules and the next four pairs seem to be rather slender, spiniferous, jointed endopodites. Whether or not gnathobases were present is not shown by the figures, but owing to the long hypostoma the appendages are grouped about the mouth. All the segments of the body, unless it were the telson, seem to have borne appendages. On the anterior end, they were clearly biramous (1912, p. 206, text fig. 10), and that they were present along the body is shown by figure 2, plate 30, 1912.
The present state of knowledge of both these peculiar animals leaves much to be desired. The indications are that the cephalic appendages are not biramous, and that only one pair of antennæ, the first, are developed as tactile organs. The thoracic appendages of Emeraldella are biramous, and also possibly those of Sidneyia. In the latter, the last two abdominal segments seem to have been without appendages, while in Emeraldella at least one branch of each appendage, and possibly both, is retained.
These animals, which may be looked upon as the last survivors of an order of pre-Cambrian arthropods, have the appearance of an eurypterid, but their dominant characteristics are crustacean. The features which suggest the Eurypterida are: elongate, obovate, non-trilobate, tapering body; telson-like posterior segment; marginal, compound, sessile eyes; claw-like third cephalic appendages; and, more particularly, the general resemblance of the test to that of an eurypterid like Strabops. In form, Sidneyia agrees with the theoretical prototype of the Eurypterida reconstructed by Clarke and Ruedemann (Mem. 14, N. Y. State Mus., vol. 1, 1912, p. 124) in its short wide head with marginal eyes, and its undifferentiated body. There is, moreover, no differentiation of the postcephalic appendages.
The crustacean characteristics are seen in the presence of five, instead of six, pairs of appendages on the head, the first of which are multisegmented antennules, and in the biramous appendages on the body of Emeraldella. It should be noted that these latter are typically trilobitic, each consisting of an endopodite with six segments and a setiferous exopodite.
Clarke and Ruedemann (1912, p. 406) have discussed Sidneyia briefly, and conclude:
It seems to us probable that the Limulava [Sidneyia and Amiella] as described are not eurypterids but constitute a primitive order, though exhibiting some remarkable adaptive features. This order possibly belongs to the Merostomata, but is distinctly allied to the crustaceans in such important characters as the structure of the legs and telson, and is therefore much generalized.
The specialization of Sidneyia consists in the remarkable development of a highly complex claw on each of the third cephalic appendages, and in the compound tail-fin, built up of the last segment and one or more pairs of swimmerets. These two characteristics seem to preclude the possibility of deriving the eurypterids from Sidneyia itself, but it seems entirely within reason that they may have been derived from another slightly less specialized member of the same order.
That Sidneyia is descended from any known trilobite seems highly improbable, but that it was descended from the same ancestral stock as the trilobites is, I believe, indicated by the presence of five pairs of appendages on the cephalon and trilobitic legs on the abdomen.
Other so called Merostomata found by Walcott in the Middle Cambrian are the genera Molaria and Habelia, both referred to the Cambrian family Aglaspidæ. These genera seem to conform with Aglaspis of the Upper Cambrian in having a trilobite-like cephalon without facial sutures, a trilobite-like thorax of a small but variable (7-12) number of segments, and a Limulus-like telson. Neither of them has yet been fully described or figured, but (Walcott 1912 A, p. 202) Habelia appears to have five pairs of cephalic appendages, the first two pairs of which are multisegmented antennæ. The thoracic appendages are likewise none too well known, but they appear to have been biramous. The endopodites are better preserved than the exopodites, but in at least one specimen of Molaria the exopodites are conspicuous.