NO. 6 MIDDLE CAMBRIAN BRANCHIOPODA, ETC. 1 73 



The appendages of the thorax are not very well preserved. They 

 indicate a leg much like that of Waptia Uddensis (pi. 27, fig. 5), com- 

 posed of broad joints, the last provided with numerous long setae. 



No appendages or setae have been observed on the four abdominal 

 segments. 



Very little is known of the interior structure, except the presence 

 of a slender, straight alimentary canal. One specimen, as viewed 

 from above (fig. 9), suggests a division into two lobes of the in- 

 terior of the head. 



Dimensions. — The largest specimen has a length of 24 mm. The 

 other dimensions as the animal is flattened in the shale are shown by 

 the figures on plate 29. 



Observations. — This species is associated with Waptia Heldensis 

 (pi. 27, figs. 4 and 5) and has the same type of body and expanded 

 caudal rami. It differs in the absence of a carapace ; in having four 

 instead of six abdominal segments ; and so far as known a different 

 form of antennae. 



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 PLENA, new species 



Plate 29, fig. 14 



This species has a proportionately larger head, thicker body with 

 shorter segments, and the caudal rami are more expanded than in the 

 associated Yohoia tenuis. A somewdiat similar form from about 75 



feet higher in the Burgess Shale (-— -j is represented by two imper- 

 fect specimens. 



Specimens of this species reach a length of 24 mm., but most of 

 them are about half as long. 



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. 



BIDENTIA, new genus 



The description of the genus is included with that of the type 

 species. 



Genotype. — Bidentia diMcilis, new species. 



