Page:Walcott Cambrian Geology and Paleontology II.djvu/197

NO. 5 two sections reduces the diameter at that point, of a specimen flattened in the shale, to one-half the cross section of the anterior portion.

Surface of anterior section marked by fine, transverse, slightly imbricating lines that serve to define narrow segments. On a specimen 55 mm. in length there are 4 lines in a distance of 5 mm.; in one 10 mm. long, 7 to 8 in 5 mm.

The posterior section has more and stronger lines than the anterior section. The distance between them varies according to the extent to which the annelid was drawn up or contracted. The distance also varies in different parts of the section, but usually they are farther apart near the ends. On a posterior section 35 mm. in length the lines on the posterior third are 1 mm. apart and on the central and anterior portion .5 mm. distant. On one specimen two rather strong hooks occur at the posterior end. No traces of a mouth or anus have been observed. One fragment shows a small enteric canal.

Dimensions.—The largest specimen of the half dozen in the collection has a length of 8 cm. A fragment of another indicates a length of 10 cm.

Observations.—This interesting species occurs in a somewhat coarser sediment than the annelids Ottoia prolifica and Miskoia preciosa with the result that the more delicate parts have not been preserved. It is hoped that the collections of 1911 may afiford better specimens.

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

Polychæta with a slender, many segmented, mobile body; small head, with well-developed eyes; parapodia on the anterior segments. Enteric canal straight; mouth and anus at the ends of the body.

One genus, Pikaia.

The description of the genus and species is united under the species.

Genotype.—Pikaia gracilens, new species.