Page:Walcott Cambrian Geology and Paleontology II.djvu/179

NO. 5 the body and quite prominent. The tail is expanded and slightly transverse at the posterior margin.

The enteric canal and traces of other internal organs show quite clearly. In the head (fig. 3) a triangular-shaped area is outlined in front, and back of it a quadrate space. Posterior to the latter are two elongate oval spaces; from the outer side of the left of these three short hooks curve inward; these were undoubtedly beside the mouth. The enteric canal begins between the bases of the two oval spaces and terminates a little beyond the center of the tail. The anus doubtless existed at this point. A transverse, somewhat irregular line at the base of the oval spaces, indicates a thin septum separating the head and body cavities.

Dimensions.—The largest specimen has a length of 20 mm. Other proportions are shown by figures 3 and 4, which represent the body flattened on the shale.

Observations.—The living representatives of this beautiful little annelid are pelagic and very active swimmers; this fact and its association in the shale with small free swimming phyllopod crustaceans indicate very clearly that it was active and free swimming. Only three entire specimens were found.

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.

Polychæta with similar segments and parapodia throughout the length of the body; retractile proboscis; straight enteric canal. Body not distinctly specialized into sections.

Body elongate, slender; with numerous uniform segments; anterior end with rows of strong setæ about the mouth. Surface of anterior portion with numerous papillæ. Parapodia abundant, branched. Proboscis elongate, retractile. Enteric canal straight, with enlargement in anterior portion.

Observations.—The long retractile proboscis with the mouth at its base (fig. 1) suggests the Class Gephyrea as does the large body