Page:Walcott Cambrian Geology and Paleontology II.djvu/283

 edge of the cephalic shield; these terminate in a minute joint having several fine setæ or spines on its margin.

On the thoracic leg two or three of the inner joints are widened out and setiferous; the outer three joints are long and slender; on the outer side of the inner joints (exopodite probably) there is a broad flabellum-like setiferous lobe, also a small oval gill (fig. 3). From beneath the posterior margin of the long abdominal segment shown in figure 4, a minute jointed leg projects on each side of the base of the telson.

Interior structure.—The alimentary canal is outlined in several specimens. On one of them (fig. 2) it extends to the posterior end of the long abdominal segment, thus indicating the position of the anal opening. Traces of the hepatic cæca beneath the head are shown by figure 3.

Dimensions.—This is a small species. The largest specimen has a length, exclusive of the telson, of 40 mm. The average length is about 18 to 20 mm. The proportions of the dorsal shield and telson are indicated by figure 2.

Observations.—I have employed above the terminology used in describing the dorsal shield of trilobites as it applies so well to this interesting species. In outward aspect the dorsal shield is essentially that of a trilobite except that there are no sessile eyes on the head shield and the posterior segments are more nearly related in form to such branchiopod crustaceans as Emeraldella brocki (pl. 30, fig. 2).

So far as can be determined, a pair of jointed legs, with an exopodite bearing a gill and flabellum, occurs on the outer side of each of the eight thoracic segments. A moderate number of specimens of the species were found in 1910 and it may be that more will be collected during the season of 1912 and that among them the cephalic appendages will be preserved. The test is thin and easily wrinkled, which obscures parts that would otherwise show through it.

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.—Habelia optata, new species.

Stratigraphic range.—The stratigraphic range is limited to a layer in a band of dark siliceous shale about 4 feet in thickness, forming a part of the Burgess shale member of the Stephen formation.