Page:Walcott Cambrian Geology and Paleontology II.djvu/266

 extending beyond the carapace. The terminal segment has from two (fig. 4) to six (fig. 5) cercopods attached to it.

A strong adductor muscle scar (adm, figs. 1 and 2) is shown on many specimens.

A pair of small pedunculated eyes project in front of the carapace, one showing on each side of a pair of minute antennules.

Appendages.—Head. Several specimens show a pair of minute, jointed antennules projecting forward from between the large jointed antennæ (fig. 2). The antennæ are large and are composed of a single stem of short joints; they may, however, be straight, unjointed, and long (fig. 2, pl. 31). I have not illustrated a specimen showing the antennules and eyes, as they were not observed until after the plates were made up.

The specimen illustrating the thoracic legs and head appendages (fig. 1) is unfortunately not so good as one which was found and cleaned of calcareous deposits after the plates were finished and before this description was written. This specimen shows three cephalic legs. The two anterior are slender (mandible and maxillula), and the posterior maxilla is large and formed of short strong joints. There are eight pairs of thoracic legs. The distal portions of these are finely shown in figure 1. The broad, setiferous joints of the exopodite are also shown near the carapace. In other specimens they extend out over the legs so as nearly to conceal them. Traces of oval gills (epipodites) are shown for three legs on the outer side of what appears to be the second joint of the leg.

Interior structure.—The alimentary canal may be traced from the anterior part of the body back to the posterior abdominal segment where it terminates between two larger cercopods of the type represented by figures 1 and 2. One specimen, not illustrated, appears to have a considerable enlargement of the canal in the head portion.

Dimensions.—The valves of the carapace average from 40 to 60 mm. in length, with other proportions as shown by figure 1.

Formation and locality.—Middle Cambrian: (35k) Extending through about 25 feet of the Burgess shale member of the Stephen formation, on the west slope of the ridge between Mount Field and Wapta Peak, one mile (1.6 km.) northeast of Burgess Pass, above Field, British Columbia.

This is a much smaller species than the other species of this genus from the Burgess shale and I am not sure that it should be referred