Page:Walcott Cambrian Geology and Paleontology II.djvu/249

 Genotype.—Opabinia regalis, new species.

Stratigraphic range.—The stratigraphic range is limited to 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 derived from Opabin, the name of a pass between Mount Hungabee and Mount Biddle, southeast of Lake O'Hara, British Columbia, Canada.

Body elongate, moderately wide, and divided into a small head section, a trunk of 16 somites, and a broad telson. The base of the head is formed of an elongated portion about as wide as long when flattened in the shale; in front of this the head narrows where the base of a large stalked eye is attached on each side. In front there is a short section from which a strong central appendage extends directly forward as viewed from above (fig. 1, pl. 28) and curves upward from the front lower side of the head when seen in profile (fig. 6, pl. 27). The appendage is narrow, wrinkled, and more or less flexible; near the anterior end it expands to form a base of attachment of a number of small, slightly incurved, short claws or spines.

The eyes are at the end of a strong, short stalk and traces of the reticulated surface of the compound eye remain on the matrix of the specimen illustrated by figure 1, plate 28.

The 16 somites of the post-cephalic body (thorax) are very uniform in appearance and size except that the posterior somites gradually decrease in size and width. Their arrangement is finely shown in profile view by figure 6, plate 27.

The terminal somite is a broad, elongate, spatulate lobe with a short point on each postero-lateral rounded angle. Between the points there is a transverse line that may mark a division of the telson and the presence of a post-anal plate.

Appendages.—The anterior, central cephalic appendage has been mentioned. It suggests the appendage of the male of the species. Reference to the possible presence of the female in the collection will be spoken of later.