Page:Proceedings of the United States National Museum, Volume 76.djvu/141

Rh through Hymenocaris, Peltocaris, Ceratocaris." Thus, while this form was classified with Apus its true position was hinted at from the beginning.

The better preserved Burgess shale specimen allows us to get a more complete conception of the major features.

Genotype.—Protocaris marshi Walcott. (See pl. 6, figs. 1, 2.)

Geographic and stratigraphic distribution.—Lower and Middle Cambrian. Vermont and British Columbia.

A single Burgess shale specimen preserving the soft body, both within the carapace and the abdominal portion that extended beyond, apparently belongs to the genus Protocaris.

Both valves of the carapace are preserved. The left one is flattened out evenly, but the right is crushed in an oblique direction. In general shape and the presence and position of a keel, this shell conforms to Tuzoia but both reticulations and marginal spines are lacking.

It was possible to remove the matrix that had filtered between the carapace and the soft body of the animal within and which preserved an impression of the underside of the left valve, thus exposing the body itself. None of the head parts are well shown, since in flattening the original convexity the anterior portion was crowded together, thereby effacing the delicate structures. Omitting the head parts the remainder of the body is divisible into a thoracic division with numerous segments all limb-bearing, an abdominal portion also composed of many segments which appear to have had much shorter or no appendages and finally a third bifurcated caudal division. This latter has practically the same shape as is Waptia fieldensis.

Comparing this species with the genotype, P. marshi, we can see but few important differences. This new species seems to have the valves more rounded in front, but P. marshi is crushed down on the sides and may thereby be somewhat changed from its original outline. The different aspect of the abdominal portions of the two species is accounted for by the fact that P. marshi was flattened vertically while P. pretiosa suffered pressure at an oblique angle.

Horizon and locality.—Middle Cambrian, Burgess shale; (loc. 35k) near Field, British Columbia.

Holotype.—Cat. No. 80483, U.S.N.M.

While the single specimen on which this genus is based leaves much to be desired, it is so unlike anything else from these beds or that has