Page:Walcott Cambrian Geology and Paleontology II.djvu/276

 hepatic cæca of Burgessia (pl. 27, figs. 1-3). The slender canal of the antennules joins the visceral space beneath the carapace at its antero-lateral angle.

Dimensions.—The average length is 15 mm. The size, proportions and relations of parts are shown by the figures on plates 25 and 26. The animal was so delicate that it was readily smashed and distorted.

Observations.—This beautiful fossil (it was called the "Lace crab" at camp) is the most abundant of the many species in the phyllopod bed. It must have swarmed in large numbers in the quiet waters.

Reference to its relations to other crustaceans will be found in the introduction.

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 generic description is included with that of the type species.

Genotype.—Nathorstia transitans, 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.

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.

The generic name is proposed in honor of Dr. Alfred G. Nathorst, the distinguished Swedish paleontologist and paleobotanist.

Dorsal shield elongate-oval in outline, very thin and delicate in structure. It may be divided into a cephalic region (cephalon), thorax, and abdomen. The cephalon is transversely semicircular with a short spine at each posterolateral angle; obscurely trilobed. One of the crescent-shaped, medium-sized eyes (e) is indicated on the right of the central axis.

Thorax faintly trilobed, composed of eight rather long (longitudinal) segments. The specimen illustrated is so flattened and crushed that it is difficult to determine the form of the segments, but other