174 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 57 



Stratigraphic range. — The stratigraphic range is limited to a band 

 of gray siliceous buflF-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. I 



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 i 

 I have dotted the approximate outline of the head. Thoracic seg- 

 ments 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 i ; 

 in another specimen, not illustrated, they are more flattened out. 



Appendages. — The only appendages of the head shown by figure i 

 are the strong antennae (a'). They have a thick, jointed basal por- 

 tion 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 setae 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. exclu- 

 sive of the telson which is about 10 mm. long. 



Observations. — At first I placed this species with Emeraldella 

 brocki (pi. 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 seg- 

 ment bearing two expanded caudal rami that form a natatory append- 

 age similar to that of Waptia Heldensis (pi. 27, figs. 4 and 5). The 

 systematic position of the genus is doubtful. It is probably a form 

 nearer the Merostomata than the Branchiopoda. 



