Page:Walcott Cambrian Geology and Paleontology II.djvu/49

 extends backward from two-thirds to four-fifths of the distance to the posterior margin; at about half its length a wing-like extension continues backward and slightly outward to a transverse margin; on each side of the terminal segment (pl. 3, figs. 2-4), and attached to its anterior side, there is a lateral swimmeret that on the inside overlaps more or less the central terminal section and on the outside margin expands so as to form, with the central terminal section, a broad caudal fin suggestive of that occurring in the Schizopoda and Decapoda; it may be that there are more than one of the lateral swimmerets on each side, but if so they are so pressed in together as not to be distinguished. The anterior margin of each segment extends under the segment in front of it from one-fifth to one-third its length at the center, the underlap gradually narrowing to where it passes from beneath the segment near its outer termination; the anterior segment passes beneath the cephalo-thorax in the same manner.

Ventral appendages: Cephalo-thorax.—Cephalo-thorax with five pairs of movable appendages. The first or antennal pair are rather stout at the base, tapering gradually until they become very slender (pl. 2, fig. 1); the joints vary in length, they are usually a little wider than long for the first half of the length of the appendage, gradually becoming proportionately longer toward the outer end. One appendage shows over thirty joints beyond the edge of the carapace, and another from its length must have many more; each joint has a short, fine spine or fringe of spines at the anterior margin of the joint. The inner point of attachment of the first joint has not been seen as it is covered by either the epistoma or cephalo-thorax in all specimens.

The second pair of appendages is formed of long, slender joints; so far as known they extend only a little distance beyond the margin of the cephalo-thorax and terminate in a joint that has two or three short spines projecting from the outer end of it.

The simple form of the third pair of appendages is shown by fig. 1, pl. 5. They have a relatively large basal joint, the anterior inner margin of which is provided with six or more sharp spines that appear to have been used in connection with the basal joints of the fourth and fifth pair of appendages as manducatory organs. This appendage has broad, strong joints in small specimens (pl. 5, fig. 1) and in large specimens it is developed into a peculiarly constructed and complex chelate terminal section; this is formed of twelve or more joints of a forward curving appendage to which