252 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



ABRUPT APPEARANCE OF THE MESONACID^ 



I will not discuss at length the question of the abrupt appearance 

 of the Mesonacidse fauna' in this paper as it will be the subject of a 

 paper on the Abrupt Appearance of the Cambrian Fauna of North 

 America/ to be read before the International Geological Congress 

 at Stockholm in August, 1910. 



I have been gradually coming to the conclusion that the most 

 natural explanation of the absence of the traces of a distinct pre- 

 Cambrian faiuia is that the North American continent in pre-Cam- 

 brian time was at such an elevation above the sea that there is 

 now no record of the sediments deposited about the continental area 

 at that time. This presupposes that the great series of pre-Cambrian 

 Algonkian sediments in the Rocky Mountain region were deposited 

 in an inland mediterranean, or a series of great lakes and flood plains 

 such as existed in Tertiary times.^ The same applies to the Lake 

 Superior, Texas, Arizona, and all of the later pre-Cambrian Algon- 

 kian formations. 



On this hypothesis the evolution of the pre-Cambrian fauna was 

 taking place in waters contiguous to the continental area, and their 

 remains, buried in the sediments then accumulating, have not been 

 found, owing to the fact that those sediments are now probably off 

 the coast lines of the continent buried beneath the sea. That such 

 a condition existed is suggested by the almost total absence of any 

 traces of life in the pre-Cambrian sediments now existing on the 

 continent. 



GEOGRAPHIC DISTRIBUTION 



Olenellus, as now restricted, has been found on both the western 

 and eastern sides of North America and the northwest of Scot- 

 land. Olenellus canadensis, O. gilherti, and O. freuionti occur in 

 the northern section of the Cordilleran Province in Alberta and 

 British Columbia, and the two latter extend far to the south in 

 Nevada and California. In the Appalachian Province 0. thonipsoni 



^ This name will be used by me in the future as the genus Olenellus is now 

 limited to the upper zone of my Olenellus Fauna of 1891 [Walcott, 1891, pp. 



515-597]. 



^This will be published as No. i of Vol. 57 of the Smithsonian Miscellaneous 

 Collections. 



^ The crustacean and annelid fossils described [Walcott, 1899, p. 238] might 

 quite as well have been fresh water as marine forms. There is nothing as far 

 as known to indicate that they were necessarily limited to a marine habitat. 



