Page:Walcott Cambrian Geology and Paleontology II.djvu/86

 The alimentary canal is large, coiled in a loose, flat spiral and divided into an oral chamber, œsophagus, stomach, and intestine, the end of the intestine opening on the ventral surface.

Specimens of the type species grew to a large size, 12 cm. in diameter. This form was gregarious and lived in large numbers in quiet waters in association with a large, free swimming crustacean fauna.

Genotype.—Eldonia ludwigi, new species.

Stratigraphic range.—Limited to a stratum of dark siliceous shale a few inches 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 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.

Body disk-formed or depressed umbrella-shaped. Exumbrella with about thirty clearly defined lobes that radiate from the center to the edge of the disk. Each lobe has a slight depression or line down the center that extends in from the outer margin from one-half to two-thirds the distance to the center (pl. 12, fig. 3). This secondary lobation gives about sixty slightly projecting, rounded lappets about the margin of the disk. In small specimens flattened sideways in the shale (pl. 11, figs. 1 and 2), the secondary lobation is emphasized so that the narrow lobes (of the 60 series) extend inward toward the center. The lobation of the exumbrella is shown by fig. 5, pl. 9; figs. 1 and 2, pl. 11; and fig. 3, pl. 12.

The surface of the subumbrella has a broad band of concentric muscle fibers that extends about half way to the center of the disk (pl. 9, fig. 5). The fibers are very fine and do not appear to be interrupted by any radiating divisions of the subumbrella surface.

From the subumbrella surface the mouth, with two short tentacles when expanded, extended downward. (See description of oral chamber and tentacles, following.)

Muscles.—Of the muscular system only the concentric muscles of the subumbrella surface have been seen, as mentioned (fig. 5, pl.