194 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 57 



hepatic cseca of Burgessia (pi. 27, figs. 1-3). The slender canal of 

 the antennules joins the visceral space beneath the carapace at its 

 antero-lateral angle. 



Dimensions. — The average length is 1 5 mm. The size, proportions 

 and relations of parts are shown by the figures on plates 25 and 26. 

 The animal was so delicate that it was readily smashed and distorted. 



Observations. — This beautiful fossil (it was called the "Lace 

 crab " at camp) is the most abundant of the many species in the 

 phyllopod bed. It must have swarmed in large numbers in the quiet 

 waters. 



Reference to its relations to other crustaceans will be found in the 

 introduction. 



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. 



Family (undetermined) 

 NATHORSTIA, new genus 



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



Genotype. — Nathorstia transitans, new species. 



Stratigraphic range. — The stratigraphic range is limited to a layer 

 in 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 proposed in honor of Dr. Alfred G. Nathorst, 

 the distinguished Swedish paleontologist and paleobotanist. 



NATHORSTIA TRANSITANS, new species 

 Plate 28, fig. 2 



Dorsal shield elongate-oval in outline, very thin and delicate in 

 structure. It may be divided into a cephalic region (cephalon), 

 thorax, and abdomen. The cephalon is transversely semicircular with 

 a short spine at each posterolateral angle ; obscurely trilobed. One 

 of the crescent-shaped, medium-sized eyes (e) is indicated on the 

 right of the central axis. 



Thorax faintly trilobed, composed of eight rather long (longitudi- 

 nal) segments. The specimen illustrated is so flattened and crushed 

 that it is difficult to determine the form of the segments, but other 



