Page:Walcott Cambrian Geology and Paleontology II.djvu/255

 The appendages of the thorax are not very well preserved. They indicate a leg much like that of Waptia fieldensis (pl. 27, fig. 5), composed of broad joints, the last provided with numerous long setæ.

No appendages or setæ have been observed on the four abdominal segments.

Very little is known of the interior structure, except the presence of a slender, straight alimentary canal. One specimen, as viewed from above (fig. 9), suggests a division into two lobes of the interior of the head.

Dimensions.—The largest specimen has a length of 24 mm. The other dimensions as the animal is flattened in the shale are shown by the figures on plate 29.

Observations.—This species is associated with Waptia fieldensis (pl. 27, figs. 4 and 5) and has the same type of body and expanded caudal rami. It differs in the absence of a carapace; in having four instead of six abdominal segments; and so far as known a different form of antennæ.

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.

This species has a proportionately larger head, thicker body with shorter segments, and the caudal rami are more expanded than in the associated Yohoia tenuis. A somewhat similar form from about 75 feet higher in the Burgess Shale $$\left(\tfrac{35k}{10}\right)$$ is represented by two imperfect specimens.

Specimens of this species reach a length of 24 mm., but most of them are about half as long.

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.

The description of the genus is included with that of the type species.

Genotype.—Bidentia difficilis, new species.