322 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 57 



DESCRIPTION OF PLATE 52 



PAGE 



Mickwitzia mitralcnsis Walcott. (See Plates 50 and 51) 310 



Fig. I. (Natural size.) Ventral and dorsal valves flattened in the 

 shale. (Locality 6ik.) U. S. National Museum, Catalogue 

 No. 60101. 



Obolclla cf. chromatica Billings 313 



Fig. 2. (Natural size.) Natural cast of both ventral and dorsal valves. 

 (Locality 6ik.) U. S. National Museum, Catalogue No. 

 60073. 



Obolella nuda Walcott 312 



Fig. 3. (X4-) Ventral valve. (Locality 6ik.) U. S. National Mu- 

 seum, Catalogue No. 60074. 



4. (X 4-) A ventral and a dorsal valve. (Locality 6ik.) U. S. 



National Museum, Catalogue No. 60075. 



5. (X 4-) Natural cast of the interior of a ventral valve. 



(Locality 6ik.) U. S. National Museum, Catalogue No. 

 60076. 



6. (X 4-) Natural cast of the interior of a dorsal valve. (Locality 



6ik.) U. S. National Museum, Catalogue No. 60077. 



7. (X 4-) Natural casts of the interior of ventral and dorsal 



valves. (Locality 6ik.) U. S. National Museum, Catalogue 



No. 60078. 

 The specimens represented by figs. 1-7 are from locality 6ik. 

 Lower Cambrian : Mahto formation ; dark, hard siliceous shale, 

 northeast base of Mumm Peak above Mural Glacier on the west side 

 of Hitka Pass, 6 miles (9.6 km.) in a direct line north of summit of 

 Robson Peak and northwest of Yellowhead Pass, in western Alberta, 

 Canada. 



