The carapace is stated to possess a large epistoma and to lack a metastoma. In eurypterids the epistoma is only known in the Pterygotus-Slimonia group. It is there a separated part of the marginal shield and originated from a part of the carapace being bent over, as shown by the direction of the sculpture. The organ figured as epistoma of Sidneyia (op. cit. pl. 5, fig. 3) is hardly comparable to the epistoma of Pterygotus, as it is not separated by sutures from the marginal shield. The view referred to is obviously a dorsal one and there seems to us much probability that the large "epistoma" may prove to be the anterior part of the carapace pushed out of position.
There is nothing visible in the figures of these finely preserved remains to suggest the presence of a metastoma. The latter is distinctly a characteristic and important organ of the order Eurypterida. If it is actually absent in the Limulava this fact would militate against the inclusion of the latter in the eurypterids.
The most important differences between the Eurypterida and Limulava appear, however, in the constitution of the abdomen. The abdomen of the Eurypterida consists invariably of six tergites on the dorsal side, to which correspond five sternites on the ventral side, and six ringlike postabdominal segments. Sidneyia, however, appears to possess as many segments on the dorsal as on the ventral side. The operculum or first ventral plate which covers both the first and second segment and which bears the genital appendage, was hence not yet developed. This would be a most primitive condition, as compared with the eurypterids, and constitutes a difference of ordinal rank. Sidneyia is further described as bearing branchiae on nine segments. The Limulava have accordingly nine abdominal segments and but three postabdominal segments, for the gill-bearing ventral segments must be movable plates of the character of sternites. This is also a difference of ordinal rank, for the number and division of segments in the Eurypterida is absolutely fixed, but if Sidneyia is in any way related to the ancestral stock of the Eurypterida it is bound to throw most interesting light on the morphogeny of the abdomen. The gills themselves must also have
