ARNOLD— THE PALEONTOLOGY AND STRATIGRAPHY OF SAN PEDRO. 29 



Resume. 



Total number of species 252 



Pelecypoda ^5 



Gastropoda 1^1 



Scaphopoda ^ 



Echiuoidea 1 



Crustacea 1 



Pisces * 



Living at San Pedro 1"2 



Percentage of fauna 68.2 



Living only north of San Pedro 15 



Percentage of fauna 6.1 



Living only south of San Pedro 36 



Percentage of fauna 14.2 



Extinct species 24 



Percentage of fauna 9.5 



Questionable 4 



Percentage of fauna 1-6 



The fauna of the upper San Pedro series as afforded by the beds near the 

 himber yard is of a character more nearly resembling that found living at the present 

 time on the coast two or three hundred miles further south. Not only is there a 

 large percentage of species now living only south of San Pedro, but of the species 

 living at San Pedro many are southern forms. Several of the northern forms 

 remained during the period of deposition of the upper San Pedro series, but in greatly 

 diminished numbers. Of the extinct forms most are new species or varieties which 

 may be found upon a more extended examination to be living. Faunal evidence 

 leads to the conclusion, therefore, that the climatic conditions on the coast near San 

 Pedro during the period of deposition of the upper San Pedro series were as warm, 

 if not warmer, than those of the present time. The change from the boreal condi- 

 tions of the upper Pliocene epoch to the tropical or semitropical conditions of the 

 Pleistocene was not sudden, but took place rather gradually, as is shown by the 

 transition fauna of the lower San Pedro deposits. 



The upper San Pedro stage described in this paper is separated from the 

 lower San Pedro deposits for the following reasons: First, there is an unconformity 

 between these upper beds and the underlying formations at all of the localities 

 examined. Secondly, the upper San Pedro beds differ lithologically from the lower 

 San Pedro strata; the former being largely gravels, while the latter are of gray sand. 

 Thirdly, the difference in the fossils of the two horizons is very marked. Many 

 found in the lower series are not found in the upper, and many of the species in the 

 latter are never found in the former. Fourthly, the upper series has a semi-tropical 

 fauna, while that of the lower series approaches the semi-boreal. 



The upper San Pedro beds do not represent the top of the Pleistocene. The 

 fauna of these upper beds, although having many species in common with the living 

 fauna of the same locality, is still quite distinct. This would suggest a period of 

 considerable length since the deposition of the strata. The number of distinctly 

 southern forms living at San Pedro during the period of deposition of the upper 

 beds also shows that there has probably been a change in climatic conditions since 



