| Sub-Class MEROSTOMATA | |||
| Order Eurypterida | Sub-Order Limulava | ||
| 1. | Cephalo-thorax long. | 1. | Cephalo-thorax short. |
| 2. | Cephalo-thorax with six (6) pairs of appendages; the anterior pair chelate antennae, and the posterior pair, long, strong swimming legs. | 2. | Cephalo-thorax with five (5) pairs of appendages; the anterior simple antennae, the third pair multi-chelate, 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 leaf-like branchial appendages. | 4. | Nine anterior abdominal segments bear leaf-like branchial appendages. |
| 5. | Surface of test with scale-like 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. |
Differences other than those tabulated will probably be found when more of the detailed structure of the Limulava can be determined.
The sub-order Limulava, as represented by the genus Sidneyia with its four pairs of cephalo-thoracic appendages and simple antennae, approaches the Trilobita, which has a similar scheme of cephalic appendages. In both, the antennae are large and simple, jointed, sensatory organs. The branchiae of Sidneyia also suggest the broad, thin joints of the exopodite of the trilobite's legs with their branchial fringes. For comparison, the branchial fringes of Neolenus serratus, a trilobite associated with Sidneyia inexpectans, are illustrated on pl. 6, figs. 1 and 2.
The branchial lamellae of Pterygotus also have branchial fringes as well as the leaf-like, oval lamellae, as illustrated by Henry Woodward.[1]
The short cephalo-thorax of Sidneyia is found also in Strabops thacheri Beecher (p. 19) from the Upper Cambrian, a form that may have had but five pairs of movable, cephalo-thoracic appendages.
- ↑ Monogr. British Fossil Crustacea, Order Merostomata, 1866-1878, pl. 11, fig. 2b; pl. 12, figs. 1a, 1d.
