Page:Walcott Cambrian Geology and Paleontology II.djvu/250

 None of the heads of the four specimens show traces of antennules, antennæ, mandibles, or maxillæ. If these appendages were large they have been broken off; if small they may be concealed beneath the crushed and flattened large posterior section of the head.

The thoracic legs are shown both in side view (fig. 6, pl. 27) and from below on a flattened specimen (fig. 1, pl. 28). They appear to be of a uniform character on all the 16 somites except the two anterior pairs, which may be smaller and have narrower terminal joints. The legs are formed of two or three rather strong, short joints followed by broad, flat, elongate-oval lobe-like joints (f, fig. 6, pl. 27). The gills are shown as oval lobes on the upper portion of the leg (br, fig. 6, pl. 27). The terminal elongate swimming joint or fin is shaped much like that of the common Branchipus vernalis Verrill. A strongly setiferous lobe occurs above the large terminal joint, but its relations to it are not clear. Another feature difficult to interpret is that of the groups of short, longitudinal lines shown in figure 1, plate 28. My present view is that they are groups of strong setæ attached to one or more of the lobe-like middle joints of the leg. I obtained an almost similar effect by pressing flat between glass plates a specimen of the recent Branchinecta paludosa (O. F. Miiller).

The details of structure of the leg cannot be determined, but judging from the material available for examination they follow somewhat closely the leg of Thamnocephalus as illustrated by Packard.

Interior structure.—The alimentary canal is readily traced from the head back to the posterior portion of the terminal lobe between the two points (fig. 1, pl. 28). Parts of the canal are convex and presumably contain portions of the matter in the canal at the time of the death of the animal.

A very beautiful specimen showing some details of the interior has recently been worked out, but with the chance of getting more satisfactory specimens before a more complete review of the Burgess shale fauna is prepared I will not attempt to interpret its somewhat confused structure.

Dimensions.—The four specimens in the collection have the following longitudinal dimensions in millimeters: