of the proboscis-like lower portion of the body. In contrast the Middle Cambrian type had a true medusa-like umbrella; concentric subumbrella muscle band; spiral subhorizontal alimentary canal, with mouth and anus off to one side of the center; and, judging from what is known of the umbrella-like body, opening at the ventral surface. The water vascular system indicated by the central ring (cr) and numerous radiating canals (rc) (pl. 8, fig. 3; and pl. 9, figs. 1 and 5), also serves to give the Cambrian form a character unlike that of Pelagothuria.
That the mouth and anus should open on the ventral surface is not unexpected, and the development of the radiate structure of the smaller canal system is also the result of the animal's gradually shifting the relations of its parts to each other, in the course of adjustment to its pelagic habitat.
The finding of a true medusa at the same locality, Peytoia nathorsti (pl. 8, figs. 1 and 2), also many free swimming crustaceans, indicates that the environment and food supply were favorable to a free swimming holothurian. The presence at the same locality of typical holothurians is very instructive, although they occur three to four feet lower down in the shales.
The specific name is given in honor of Dr. H. Ludwig, who has done such splendid work on the holothurians dredged by the Albatross.[1]
Formation and locality.—Middle Cambrian: (35k) Burgess shale[2]
- ↑ Mem. Mus. Comp. Zool., Vol. 17, 1894, No. 3, pp. 1-184, pls. 1-19.
- ↑ Burgess Shale
This name is proposed as a geographic name for a shale to which the term of Ogygopsis shale was given in 1908 (Smithsonian Miscellaneous Collections, Vol. 53, p. 210). It is proposed to call it the Burgess shale of the Stephen formation.
Type locality.—Burgess Pass east of Mount Burgess and on the west slope of Mount Field and the ridge extending to Wapta Peak. About 3000 feet above and from three to live miles on the trail from the town of Field on the Canadian Pacific Railway, British Columbia, Canada. The Burgess formation occurs to the southward across the Kicking Horse Canyon in the side of Mount Stephen.
Derivation.—From Burgess Pass, the type locality.
Character.—Argillaceous, calcareous, and silico-argillaceous shales.
Thickness.—On the west slope of Mount Field, 420 feet; on the northwest slope of Mount Stephen, about 150 feet.
Stratigraphic position.—Above thin bedded, dark gray, and bluish-black limestones of the Stephen formation, and beneath a thin bedded limestone
