The description of the branchiae will be found under the description of Sidneyia, the typical genus of the Limulava.
Observations. The suborder Limulava differs from the Eurypterida, to which it is most nearly related, in having a large epistoma similar to that of the Trilobita; in not having a metastoma, chelate antennae, and swimming cephalic appendages; and in having a broad, fan-shaped caudal fin, and branchial appendages more or less unlike the lamellar branchiae of the Eurypterida and Xiphosura.
The relations of the order and suborder are shown in the following tabular view:
| Subclass MEROSTOMATA | ||||
| Order Eurypterida | Suborder Limulava | |||
| 1 | Cephalothorax long | 1 | Cephalothorax short | |
| 2 | Cephalothorax with six (6) pairs of appendages; the anterior pair chelate antennae, and the posterior pair, long, strong swimming legs | 2 | Cephalothorax with five (5) pairs of appendages; the anterior simple antennae, the third pair multichelate, and the posterior pair short, the outer joint serving as a branchial organ | |
| 3 | Epistoma present in Pterygotus where it is narrow. Metastoma large | 3 | Epistoma large. Metastoma unknown | |
| 4 | Six anterior abdominal segments bear leaflike branchial appendages | 4 | Nine anterior abdominal segments bear leaf like branchial appendages | |
| 5 | Surface of test with scalelike ornamentation | 5 | Surface of test smooth or with imbricating lines, as in many of the Trilobita | |
| 6 | Terminal segment a simple lanceolate or spatulate telson | 6 | Terminal segment a caudal fin formed of a central expanded telson and one or more swimmerets on each side | |
In view of the unusual structures here set forth it is germane to this work to inquire whether these Limulava should be construed as belonging to the Eurypterida in the capacity of a suborder or if they may not more properly assume a higher grade as an independent and coordinate group.
The Eurypterida, as represented by the genera hitherto referred to them, constitute a very compact order with well fixed characters. If
