HORIZON OF ArCELLA CRASSICOLLIS 391 



cbanieter of the other forms is suggestive of the older Jurassic faunas of the 

 Taylorsville region, but it must be admitted that there is no definite evidence 

 of this." 



The following list, prepared by Doctor Stanton, contains all the shells 

 yet discovered in the plant beds at Oroville : 



Ostrea sp. 



Pecten. Two or three species. 



Aucella ? sp. Two imperfect specimens with the form of A. pioclii or 

 A. errinytoni. 



Modiola sp. 



Trigonia sp. 



Gardium ? sp. The abundant species that in a previous report was 

 thought to be an aviculoid shell, and doubtfully referred to Eumi- 

 crotis. The specimens in the present collection are somewhat bet- 

 ter preserved, but the generic reference is still doubtful. 



Belemnites sp. Several fragments. 



Horizon of Aucella crassicollis 



The stratigraphie relations of the beds containing Aucella crassicollis 

 are more clearly and extensively exposed along the western side of the 

 Sacramento valley than anywhere else in California and Oregon. 

 Although that region has not yet been mapped in detail because there is 

 no appropriate topographic base, it has been studied with care for its 

 structure and fauna* and a number of sections have been measured. 



The Cretaceous rocks (Shasta series and Chico) have a total thickness 

 on Elder creek, in Tehama county, of about 30,000 feet, of which the 

 lower 20,000 belongs to the Knoxville, overlain by 6,000 feet of Horse- 

 town and 4,000 of Chico. The various forms of Aucella, excepting Au- 

 cella erringtoni, which occurs in the Mariposa, have been recognized in 

 the Knoxville and are considered by Doctor Stanton as two distinct spe- 

 cies — Aucella piochi and Aucella crassicollis. Although the two forms 

 sometimes occur near together, in general the strata containing Aucella 

 piochi lie Ijeneath those in which Aucella crassicollis is most abundantly 

 developed, and the zone of the latter is apparently limited to the upper 

 2,000 feet of Knoxville beds, where it is associated with other forms which 

 are characteristic of the same horizon. Elsewhere in California and 

 Oregon the Aucella crassicollis zone may vary greatly in thickness, Init 

 wherever it occurs in determinable order it forms the top of the Knox- 

 ville next in succession below the base of the Horsetown. This general 



« The Sliasla-Chico series, by .7. S. Diller and T. W. Stanton, Bulletin of the Oeological 

 Society of America, vol. 5, pp. 4.S5-467. and Contributions to the Cretaceous paleontology 

 of the 1 aclfic coast : The fauna of the Knoxville beds, U. S. Geological Survey Bulletin 

 no. 133, by T. W. Stanton. 



