Page:Walcott Cambrian Geology and Paleontology II.djvu/261

 leg when flattened so as to bring this feature in profile. These triangular expansions also show on the fourth and fifth joints of some specimens. One specimen shows on seven pairs of legs, small, elongate, oval bodies attached near the first joint to the outer side of the leg. These bodies left but slight impression on the rock and are rarely seen. They appear to represent the gills.

A pair of minute, jointed, setiferous appendages projecting from beneath the first abdominal segment suggests the presence of a simple phyllopodan natatory leg. The remaining thirty or more segments of the abdomen and telson are limbless so far as can be determined from many specimens.

Interior structure.—The thin carapace has preserved and now shows most beautifully the large hepatic cæca. The position and connection of these is finely shown at (kd) by figures 1 and 2, plate 27. The alimentary canal is large, expanded in the head as a stomach (st, fig. 1), and extending directly through the body from the front of the head to the first abdominal segment where it presumably terminated at the anus.

Dimensions.—The average length of the larger specimens of the carapace is about 10 mm. Some are 12 mm. and many 6 to 8 mm. The proportions of carapace, thorax, and abdomen are fairly well shown by figure 1, except that the long, thin abdomen continues backward until it exceeds the entire length of head and thorax about 3 to 2, or by actual measurement in one example, 21 mm. to 12 mm. for the head and body respectively.

Observations.—The very delicate carapace resembles that of the recent Lepidurus and like the latter takes many forms when flattened by pressure. An illustration of the deformation of the carapace of Burgessia is given by figures 3 and 4, plate 30. I at first thought that the latter represented quite a different form from Burgessia bella, but with the examination of many specimens a fine series was selected, showing gradations between the typical specimens on plate 27, figures 1 and 2, and the crushed side views shown by figures 3 and 4, plate 30. I had selected many specimens to be photographed but decided to illustrate only five in this preliminary paper as many interesting points have come up that more material may throw light on.

Among living Branchiopoda the Apodidæ furnish the most suggestive examples for comparison with Burgessia bella. The absence of abdominal segments with appendages is a marked distinction, also the presence of eight pairs of thoracic legs. The long slender