Page:Walcott Cambrian Geology and Paleontology II.djvu/96

 The presence of so highly organized a medusa in the central part of the Middle Cambrian terrane is not surprising in view of the numerous traces of Medusæ in strata of Lower Cambrian age.

Genotype.—Peytoia nathorsti, new species.

Stratigraphic range.—Limited to a stratum of dark siliceous shale 2 feet in thickness in the lower portion of the Ogygopsis zone (= Burgess shale) of the Stephen formation as described in 1908. (See footnote on page 51 of this paper.)

Geographic distribution.—On the west 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.

Of this medusa we have three specimens of the impression made by the subumbrella lobes. The flattened disk has a broadly elliptical outline with the outer margin slightly indented where the outward curving ends of the lobes unite so as to indicate very short, rounded lappets.

There are thirty-two lobes arranged in a quadrate series. This includes four large lobes, one extending outward on each side of the quadrate central opening, and seven narrow lobes between the broad lobes in each quadrant. The inner ends of the lobes terminate so as to form a quadrate opening with one of the broad lobes at the center of each side. Each lobe has two short, broad points that project inward a short distance. These points appear to have been the points of attachment of the parts about the mouth, or possibly oral arms.

No traces of a concentric muscle band.

A few radial lines parallel to the margins of the lobes serve to define a narrow band on each side of each lobe. A trace of the canal system of the subumbrella is shown by the cast of small anastomosing canals extending out on some of the radial lobes to the outer margin.

Dimensions.—The largest disk has a diameter of 63 mm. on its longer axis, and 51 mm. on the shorter. The central quadrate opening is 21 by 17 mm., exclusive of the projecting points.

Observations.—The three specimens of this species occur in partings of the siliceous shale in association with annelids and crustaceans that indicate that they were deposited on the bottom in quiet water, and were not left on a beach between tides. The subumbrella disk