32 CHARLES D. WALCOTT 



Although this fauna, according to our present knowledge, is the 

 oldest known Cambrian fauna, it includes representatives of the 

 several classes of invertebrates which I will enumerate. 



Actinozoa. — The corals are represented by a very primitive form 

 of Prolopharetra, a small form of cup-shaped Archaeocyaihus, and a 

 small Ethmophyllum closely allied if not identical with Ethmo phyllum 

 whitneyi (Meek),' which occurs higher in the section. The latter is 

 not a notably simple or primitive form of the Archaeocyathinae; on 

 the contrary, it is nearly as far advanced as any species known in the 

 Cambrian. 



Vermes. — The annelid borings and trails that occur in and on the 

 sandstones and shales are much like those of the Middle and Upper 

 Cambrian. 



Molluscoidea. — The two species of brachiopods represent widely 

 separated genera. Mickwitzia occidens Walcott^ is one of the primi- 

 tive forms of the Paterinidae, while Trematoholiis excelsis Walcott^ is 

 a typical form of the Siphonotretidae. The interval represented by the 

 relative development of Mickwitzia and Trematoholus is sufficient to 

 convince us that we must look far back in Cambrian, or it may be pre- 

 Cambrian, time for the progenitors of the inarticulate brachiopods. 



Pteropoda. — The forms representing Orthotheca are abundant, 

 large, strong, and evidently as well developed as those of the Middle 

 Cambrian. 



Crustaceans. — The trilobites thus far found at this horizon are 

 contined to one species of the genus Holmia. Nevadia iveeksi, new 

 species (referred at first to Holmia), has many segments, and is more 

 primitive than such forms as Olenellus thompsoni Hall-* and Holmia 

 hroggeri (Walcott)^ of the upper portions of the Lower Cambrian 

 section. The other species, Holmia roivei, new species, is of the same 

 general type as Holmia brbggeri. The absence of all other trilobite 

 genera is the most marked feature of this early Cambrian fauna. 



1 E. gracile is considered to be a synonym of E. li'hitneyi {Bull. U. S. Geol. Survey, 

 No. 30, 1886, pp. 81-84). 



2 Smithsonian Miscellaneous Collections, Vol. LIII, No. 3, 1908, p. 143. 



3 Ibid., p. 146. 



4 See Bull. U. S. Geol. Survey, No. 30, 1886, p. 167. 



5 See Tenth Annual Report, U. S. Geol. Survey, 1891, p. 638. 



