Stratigraphic range.—The stratigraphic range is limited to a band of dark siliceous shale about 4 feet in thickness forming a part 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.
Generic name derived from Pika, name of a mountain peak northwest of Laggan, Alberta, Canada.
PIKAIA GRACILENS, new species
Plate 20, figs. 1 and 2
Body elongate, slender, and tapering at each end. It is formed of many segments that are defined by strong annular shiny lines. Head small with two large eyes and two tentacles as shown by fig. 1. Back of the head the first five segments carry short parapodia that appear to be divided into two parts.
The enteric canal extends from end to end without change in character. It is relatively large along the central portions and tapering toward the ends. Judging from such specimens as the one illustrated by fig. 2, its annulations correspond in size with those of the body.
Surface apparently smooth. Two entire adult specimens and several fragments of others indicate a length of about 5 cm.
Observations.—This was one of the active, free-swimming annelids that suggest the Nephthydidæ of the Polychæta. I am unable to place it within any of the families of the Polychæta, owing to the absence of parapodia on the body segments back of the fifth. As compressed in the shale the study of a number of specimens of the posterior portion of the body leads me to think that it may have been flattened and thus been a much more effective aid in swimming.
Formation and locality.—Middle Cambrian: (35k) 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.
OESIA, new genus
Polychæta with thin, translucent, irregular tube having an enlarged head region. Segments numerous and more or less distinctly shown for the entire length. Enteric canal small and extending the length of the body. Hooks of anterior region very small.
