Page:Walcott Cambrian Geology and Paleontology II.djvu/258

 carapace was very thin and is now frequently wrinkled and folded in a manner resembling pressed and dried specimens of the carapace of the recent Lepidurus glacialis.

The two parts of the carapace appear to be attached along the longitudinal median line to the dorsal surface of a number of the segments of the head and thorax. The line between the two parts of the carapace appears to be at about the third thoracic segment of the body. There is nothing in the appearance of the cephalic carapace to indicate how many segments are coalesced in it, but on one specimen of a posterior part 14 segments are faintly indicated. Whether these are only the impressions of the underlying segments or represent coalesced segments I am not prepared to state.

The body is slender and composed of several cephalic segments, probably 5, and 17 to 19 thoracic segments. Three of the latter appear beneath the anterior part of the carapace, 14 beneath the posterior part, and two extend beyond the posterior edge of the carapace. An abdomen is indicated by two small segments and a short, slender-jointed telson-like extension (fig. 4).

Appendages.—In the head of Burgessia bella (pl. 27, fig. 3) the cephalic appendages are all anterior to the lateral canals connecting the hepatic cæca and alimentary canal. Specimens of Naraoia compacta show the hepatic tubes, and anterior to it the outline of four divisions or segments of the central axis of the head. What may be the outer end of a simple straight antenna projects from the side of the carapace, and seventeen legs extend from beneath the carapace in figure 4. Of these, three are referred to that portion of the body beneath the anterior part and 14 to the posterior part of the carapace. The legs have long, slender joints, all of which except the distal have a strong fringe of fine setæ. The legs terminate in a minute, slightly curved claw. I have not seen a flabellum or gill in position, but considerable evidence of their presence along the side of the body is furnished by faint outlines showing through the carapace.

Interior structure.—The large hepatic cæca are beautifully shown in the sides of the anterior half of the carapace (fig. 4) also the canal connecting with the alimentary canal. The latter canal is finely shown in the thorax, where it extends to the posterior segment a little back of the posterior margin of the carapace, where the slender telson joins the body.

Observations.—This species furnishes another interesting addition to the group of Middle Cambrian Branchiopoda from the Burgess shale. It is essentially Burgessia-like (pl. 27, figs. 1-3) with the