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MUSEUM BULLETIN NO. 2. 
Wanneria, and Holmia ) that have been placed 1 upon the theoret- 
ical line of evolution between the primitive Nevadia and the 
Middle Cambrian Paradoxidae; (c) there is widespread evidence 
of a transition fauna (the Albertella ) between the Lower and 
the Middle Cambrian, a fauna distinct from its predecessors 
in the region and so closely united with those which follow 
that it has in this paper been referred to the Middle Cambrian, 
but whose very presence betrays a kinship between the two 
divisions of the Cambrian which is only emphasized by the 
apparent recurrence in younger strata of a surviving member of 
the illfated Mesonaeidse; and (at) nowhere have unconformable 
relations between the two units been observed. To be sure 
there is evidence of a gradual encroachment upon an eastern land 
mass of the waters of a great Lower-Middle Cambrian sea which 
appears to have been continuous from Nevada to British Colum- 
bia, but the slowness of the approach effectually barred the 
Mesonacidse from participating in the march upon the promised 
land and reserved to their descendants the peopling of shores 
not 50 miles away. 
In California and Nevada the Lower Cambrian arenaceous 
series contains large quantities of calcareous and argillaceous 
matter, and Olenellus and its congeners there yield our only 
record of their existence during a period long enough for the 
deposition of a mile or more of strata. Here the time interval 
necessary for the gradual spread of the Lower Cambrian sea 
upon the western portion of our continent was spent under 
conditions of more or less stable equilibrium and the deposits 
have yielded representatives of the Mesonacidae ranging from 
the most primitive ( Nevadia ) to the most highly specialized 
( Olenellus s.s.) 2 Elsewhere in the United States portion of the 
Cordilleran region the members of this group appear to be 
confined to Olenellus (s.s.) and occur only in the upper thin- 
bedded layers of a highly arenaceous series referred to the 
Lower Cambrian, 3 as if the waters in or by which these sediments 
were deposited proved so inhospitable that the life which teemed 
1 Walcott: Smithsonian Misc. Coll., vol. 53, No. 6, 1910, p. 249. 
2 Idem. 
8 The Middle Cambrian age of a lithologic equivalent of this series has been 
described, p. 102. 
