162 BRUCE CLARK 
What may be considered as the type section of the Siphonalia 
sutterensis zone is found at Table Mountain near Oroville, Cali- 
fornia. Dickerson™ also referred the Eocene of the Marysville 
Buttes to this horizon, and later the fauna of the Umpqua forma- 
tion of the Roseburg Quadrangle was included in the same horizon.? 
He recognized the distinctiveness of the Siphonalia sutterensis 
fauna from that of the type Tejon. The absence of many of the 
highly ornamented molluscan species, so common in the typical 
Tejon, and the presence of an equally large number of species in 
the former fauna not found in the latter, appear to be the principal 
reasons for his belief that the two faunas were not contemporaneous. 
Dickerson attempted to establish the stratigraphic sequence of 
his upper faunal zone in relation to that of the typical Tejon 
indirectly, not having the two faunas in the same section. His 
idea that the Siphonalia sutterensis fauna is younger than that 
of the typical Tejon appears to have been founded principally 
upon what he considered evidence for different stages of evolution of 
certain pelcypods, such as Venericardia planicosta merriami Dicker- 
son and Cardium marysvillensis Dickerson. He believed that the 
variety merriami was derived from the variety hornii. Later 
stratigraphic work has shown that these species occur in a sequence 
the reverse of that which Dickerson originally supposed, the 
Venericardia planicosta merriamt coming in beds older than those 
containing the variety hornii. The same is true of the other species, 
which were derived from typical Tejon species. 
Another line of evidence which was presented as a basis for 
believing that the fauna of the Siphonalia sutterensis zone is 
closely related to that of the typical Tejon and therefore should 
be classed as Tejon, is the presence of a large percentage of species 
in the former fauna which, according to his determination, are 
also present in the typical Tejon. The writer has had access 
to all the collections which Dickerson had when he came to the 
foregoing conclusions. Their study has shown that there is a 
t Op. cit., pp. 403-6. 
2R. E. Dickerson, ““The Fauna of the Siphonalia Sutterensis Zone in the Rose- 
burg Quadrangle, Oregon,” Proc. Cal. Acad. Sci., Vol. IV (1914), pp. 113-28, 
Pls. 11-12. 
