Page:Walcott Cambrian Geology and Paleontology II.djvu/254

 Stratigraphic range.—The stratigraphic range is limited to a band of dark siliceous shale about 4 feet in thickness forming a part of the Burgess shale member of the Stephen formation.

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.

The generic name is derived from Yoho, the name of the beautiful Yoho Valley, east of Mounts Wapta and Field.

Observations.—Two species have been referred to the genus, Y. tenuis and Y. plena. Both are elongate, slender, and have a small cephalic carapace, eight thoracic and four abdominal segments, with expanded caudal rami on the posterior segment.

Body elongate, slender. Head short and without a carapace. Thorax with eight segments. Abdomen with four segments, the posterior bearing a pair of expanded caudal rami.

Head sub-quadrangular in outline, composed of five coalesced segments, the posterior four of nearly equal width and the anterior narrow. The segmentation of the head is very plainly shown on some specimens (fig. 12) and not on others (fig. 10). I have inclined at times to consider that there was a cephalic carapace, but finally decided that if present it was very small and thin and not to be recognized as a true carapace. The eyes are small, pedunculated, and rarely seen, since they appear, in side view, to be in a niche between the first and second segments of the head. As seen from above, on a specimen from which the edge of the test has been removed, they are small, round, bright spots (fig. 9).

The thorax is composed of eight segments that, with triangular-shaped pleurons on each side, clearly separate the thoracic segments from the four cylindrical segments of the abdomen. The two expanded rami attached to the posterior abdominal segments were thin and readily distorted by compression in the shale.

Appendages.—The first pair (antennules) appear to be short and blunt as they project beyond the anterior end of the head (fig. 13, side view; fig. 9, top view). The second pair (antennæ) have several joints (three are shown beyond the margin of the head) with a terminal group of three long, slender, curved spines (fig. 13). These probably represent the claspers of the male. The third, fourth, and fifth cephalic appendages show as small jointed legs hanging below the head.