Stratigraphic range.—The stratigraphic range is limited to a band of gray siliceous buff-weathering shale forming a part of the upper portion of the Burgess shale member of the Stephen formation.
Geographic distribution.—On the slope of the ridge between Wapta Peak and Mount Field, north of Burgess Pass, and about 3800 feet above Field on the line of the Canadian Pacific Railway, British Columbia, Canada.
The generic name is derived from Bident, the name of one of the mountain peaks east of the "Valley of the Ten Peaks," south of Laggan, Alberta, Canada.
BIDENTIA DIFFICILIS, new species
Plate 30, fig. 1
Body elongate, with well-marked head, thoracic segments, and expanded caudal rami. Head short and, as pressed flat on the shale, semicircular with the straight side jointed to the thorax. In figure 1 I have dotted the approximate outline of the head. Thoracic segments short and of nearly equal length; there appear to be eleven that have attached appendages. Abdomen with one segment and a pair of expanded rami. The latter are pressed together in figure 1; in another specimen, not illustrated, they are more flattened out.
Appendages.—The only appendages of the head shown by figure 1 are the strong antennæ (a'). They have a thick, jointed basal portion with two long jointed branches. The latter may be the claspers of the male.
The thoracic limbs are obscure owing to the great pressure and flattening they have undergone. Those best preserved along the central segments show a large, broad lanceolate terminal segment fringed with long setæ on the posterior margin; gill lobes are indicated on the upper portion of the leg.
Dimensions.—The largest specimen has a length of 45 mm. exclusive of the telson which is about 10 mm. long.
Observations.—At first I placed this species with Emeraldella brocki (pl. 30, fig. 2), but further study of the specimen illustrated and one other led to its separation as the type of a new genus and species. It differs from E. brocki in having an abdomen of one segment bearing two expanded caudal rami that form a natatory appendage similar to that of Waptia fieldensis (pl. 27, figs. 4 and 5). The systematic position of the genus is doubtful. It is probably a form nearer the Merostomata than the Branchiopoda.
