Page:Walcott Cambrian Geology and Paleontology II.djvu/46

 Abdomen with twelve segments, the anterior nine of which have a pair of ventral appendages to which the branchiae are attached; the posterior segment has a central spatulate-shaped section that, combined with swimmerets, forms a strong caudal fin.

The description of the branchiae will be found under the description of Sydneyia, the typical genus of the Limulava.

Observations.—The sub-order 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.

Cephalo-thorax small, without lobes, eyes marginal; ventral side with large epistoma, five pairs of movable appendages, the gnathobases of the three posterior pairs forming organs of manducation. Abdomen twelve-jointed, the three posterior segments annular and narrow, the terminal one forming, with lateral swimmerets, a fan-like tail; nine anterior segments with a pair of ventral branchial appendages on each; the three posterior segments without ventral appendages. Surface smooth or ornamented by narrow, irregular, fine, imbricating ridges.

Body elongate, broadly oval in outline, attaining as now known a length of 17 cm., covered with a thin dorsal shield or crust, divided into a short cephalon, broad anterior abdomen, and narrow posterior abdominal portion. Cephalo-thorax transverse, short, depressed convex, as compressed in the shale, with broadly rounded frontal margin and antero-lateral angles; margins smooth. Eyes reniform and situated near the postero-lateral outer margin. Ventral side with five pairs of movable appendages. The anterior pair are large, long, simple, jointed antennae; second pair, slender and jointed; third pair with numerous spines on the front side of the joints and with variously developed chelate-like outer joints (see pl. 4) ; fourth pair, slender and jointed; fifth pair with a large basal joint, and an outer, broad joint or palp that is fringed with fine branchial setae or spines. A large epistoma is attached to the front margin and back of it the gnathobases of the appendages, the three posterior pairs of which form the organs of manducation.