Page:Walcott Cambrian Geology and Paleontology II.djvu/280

 There are no indications of eyes on the cephalon. A narrow median longitudinal ridge occurs on the posterior half of each of the seven segments of the thorax and on an eighth segment that appears to be still attached to the pygidium.

This little species, 16 mm. in length, appears to be generically identical with Mollisonia symmetrica, which occurs at about the same geological horizon a few miles distant. It is much smaller and more slender than that species.

Formation and locality.—Middle Cambrian: (35k) Burgess shale member of the Stephen formation (phyllopod bed), on the west slope of the ridge between Mount Field and Wapta Peak, one mile (1.6 km.) northeast of Burgess Pass, above Field, British Columbia.

Of this species there are several fragmentary specimens. The species differs from M. gracilis, with which it is associated, in the character of the thoracic segments and pygidium; also, so far as we can determine from this superficial study, there are seven segments and the pygidium shows distinct segmentation with a denticulated border.

The specimen illustrated by fig. 6 indicates that the species was the largest of the genus and may have had a length of from 5 to 6 cm.

Formation and locality.—Middle Cambrian: (35k) Burgess shale member of the Stephen formation (phyllopod bed), on the west slope of the ridge between Mount Field and Wapta Peak, one mile (1.6 km.) northeast of Burgess Pass, above Field, British Columbia.

The generic description is included with that of the type species.

Genotype.—Tontoia kwaguntensis, new species.

Stratigraphic range.—The stratigraphic range is limited to the upper portion of the Tonto sandstone on the surface of a few thin layers of sandstone.

Geographic distribution.—The species has been found only in Kwagunt Valley off the eastern end of Kiabab Plateau in the Grand Canyon of the Colorado, Northern Arizona.

The generic name is derived from Tonto, an Indian name applied to the basal sandstone of the Cambrian series in the Grand Canyon region.