NO. 6 MIDDLE CAMBRIAN BRANCHIOPODA, ETC. I49 



also informs me that Chapter XIII, which includes the Holothu- 

 rioidea, was written by E. S. Goodrich, as stated in a footnote on 

 page 218, Part III, of Lankester's Treatise on Zoology. This was 

 also overlooked in citing from that portion of the treatise. Due 

 credit will be given in the final paper on the subject of the Burgess 

 shale fauna. 



HABITAT 



The crustaceans now found in the Burgess shale lived in quiet, 

 relatively shallow waters swarming with life and readily accessible to 

 the fauna of the open sea. In the preliminary study of the fauna 

 I have distinguished 56 genera in collections from a block of shale 

 not over 6 by 40 feet in area and 7 feet in thickness. Individuals of 

 several species of crustaceans occur in large numbers at three hori- 

 zons, notably Marrella splendens and Hymcnocaris perfecta. Trilo- 

 bites, with the exception of the genera Agnostus and Microdiscus, 

 are not abundant, although their tests almost make up calcareous 

 shales a few feet below the base of the Burgess shale. 



The compact, smooth, exceedingly fine-grained siliceous Burgess 

 shale was deposited from relatively quiet, muddy water. At inter- 

 vals this condition must have continued for some time as layers of 

 the shale several inches in thickness have the crustaceans distributed 

 irregularly through them. Where the shale is in thin layers with 

 distinct lamination and bedding surfaces the fossils are more abun- 

 dant but less perfectly preserved. The presence of carbonic acid 

 gas at the surface of the mud has already been spoken of.^ 



Owing to faulting and alteration of the shales by shearing the area 

 available for collecting is limited to about 120 feet of outcrop on a 

 steep slope of the mountain. This condition limits our information 

 as to the original extent of this remarkable mud deposit. It was 

 probably laid down in a small bay or lagoon in close connection with 

 the shallow Middle Cambrian sea. 



CHARACTER OF THE SHALE 



Mr. E. S. Larsen, Jr., of the United States Geological Survey, 

 examined sections of the shale and from his notes the following is 

 taken : 



The microscopic examination of the thin section of the rock shows that it 

 is very fine-textured— so fine that much of the material shows aggregate 

 polarization. It is made up largely of white mica, which occurs in minute 

 shreds or scales arranged parallel to the cleavage of the rock. Kaolinite is 



Smithsonian Misc. Coll., Vol. 57, No. 3, 191 1> P- 42. 



