Page:Walcott Cambrian Geology and Paleontology II.djvu/281



This unusual form has a convex dorsal shield divided into a cephalon, thorax, and pygidium. The cephalon has a narrow, raised margin with the lateral third on each side of the cephalon rising rapidly to the elevated central section. The central section is slightly flattened and has a sharp median ridge extending from the anterior end back to the posterior margin. A similar ridge crosses each thoracic segment and extends back on the pygidium about one-fourth its length.

The segmentation of the cephalon is suggested by two narrow ridges crossing the right lateral space beneath the elevated central portion.

There is a suggestion of the presence of eyes on the posterior outer edge of the elevated central portion of the cephalon.

The thorax is divided into four segments by rounded, elevated, narrow ridges as shown by figure 4.

The pygidium is shorter and smaller than the cephalon, but I think this may be owing to the breaking away of the margin.

There is only one specimen and that is a matrix in a fine, hard sandstone. Trails that appear to have been made by this or a similar form occur on the surface of several of the layers of sandstone adjoining the one on which the specimen illustrated occurred.

I am not at all sure that this species should be placed with the trilobites, but with such forms as those illustrated by figures 3 and 5 at the same geological horizon it seems best to classify it with them for the present.

The total length of the specimen illustrated is 25 mm. Other proportions are shown by figure 4.

Formation and locality.—Middle Cambrian: (73) Tonto sandstone, upper portion, Kwagunt Valley, Grand Canyon of the Colorado, Northern Arizona.

Body elongate, transversely trilobed. Cephalo-thorax with or without sessile eyes; on the ventral side it has an epistoma and five pairs of movable appendages.

Thorax with 8 to 11 segments, each of which has a pair of jointed appendages. Abdomen with 1 to 3 segments.

One family, Aglaspidæ Clarke.