17.2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 57 



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. 



YOHOIA TENUIS, new species 



Plate 29, figs. 7-13 



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 seg- 

 ments, 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 seg- 

 ments from the four cyHndrical segments of the abdomen. The 

 two expanded rami attached to the posterior abdominal segments 

 were thni 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 (antennae) 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. 



