NO. 6 MIDDLE CAMBRIAN BRANCHIOPODA, ETC. I7I 



gested. The large opening- on the side of the head indicates a 

 large pedunculated eye comparable with that of Opahinia regalis (pi. 

 28, fig. I). 



Appendages. — Of the head appendages, the antennse are the best 

 preserved. These are large and composed of several strong joints 

 of which three now show from beneath the carapace ; the second of 

 these bears a long slender branch on its inner margin, and the third 

 two branches, one of which is similar to that of the second joint. These 

 two branches appear to be composed of one very long slender joint fol- 

 lowed at the end by several very short small joints that curve up- 

 ward and presumably gave the branches flexible extremities ; the third 

 and lower branch has a similar slender proximal joint that at its outer 

 end has three slender, jointed branches. This structure makes a 

 very effective clasper of each of the antennse. Back of the right 

 antenna are two narrow appendages that may be the ends of one of 

 the third and fourth pairs of head appendages. 



The thoracic legs terminate in flat, elongate, broad, lanceolate 

 joints. The terminal joint is about three-fifths the entire length of 

 the leg, and has a fringe of strong setse on its outer and posterior mar- 

 gin. The condition of preservation is such that the details of struc- 

 ture of the other portions of the leg cannot clearly ba determined. 



The size and proportions of the type and only example of the 

 species are shown by the lower specimen of figure 6, plate 31. 



Observations. — This is one of the rare species in the collection. 

 The anterior half was found after a dynamite blast and later the 

 matrix showing the posterior portion and part of the anterior was 

 picked out of the debris. Working as we often did with cold rain 

 or snow falling, fragments once lost trace of were rarely recovered. 



The large natatory, distal joints of the thoracic legs are much like 

 those of Opabinia regalis (fig. 6, pi. 27), also the large eye. For the 

 present the species is placed in the family Opabinidse, although I 

 fully realize that the reference is of the most tentative character. 



Formation and locality. — Middle Cambrian: (35k) Burgess shale 

 member of the Stephen formation, 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. 



YOHOIA, new genus 

 The description of Yohoia tenuis embodies the characters of the 

 genus. 



Genotype. — Yohoia tenuis, new species. 



