20 CALIFORNIA ACADEMY OF SCIENCES. 



ECHINOIDEA. 



Echinararhniii.i excentrkux, P St.rongijlocentrotuiifranciscann»,'P StroiKjylocentrolnapurpvratns, P 



CRUSTACEA. 



Cancer breiveri, E 



Resume. 



Total number of species 247 



Peleoypoda, species 74 



Gastropoda, species 165 



Scaphopoda, species 4 



Echinoidea, species 3 



Crustacea, species 1 



Species living at San Pedro 158 



Percentage of fauna 64 



Species living only north of San Pedro 43 



Percentage of fauna 17.4 



Specimens living only south of San Pedro 8 



Percentage of fauna 3.2 



Species extinct 31 



Percentage of fauna 12.5 



Species of questionable habitat 7 



Percentage of fauna 3 



This is also a cold water fauna. Not only is there a large percentage of 

 species which are found living only north of San Pedro at the present time, but of 

 those species in the fauna which are now living at San Pedro, a great many are 

 northern forms. A few distinctly southern forms are found in the fauna, however, 

 which shows that the cold climatic conditions prevalent along this part of the coast 

 during the upper Pliocene times were beginning to give place to more temperate con- 

 ditions in the early part of the Pleistocene. The fauna of the lower San Pedro series 

 is a transitional one between the boreal fauna of the Pliocene and the warm water 

 fauna of the up})er San Pedro series. The climatic conditions were therefore chang- 

 ing during lower Pleistocene times; and the climate at the end of the period of de- 

 position of the lower San Pedro deposits was much warmer than that at the end of the 

 period of deposition of the Pliocene. The period of denudation between the Pliocene 

 and Pleistocene epochs was one during which the conditions were also changing. 



The lower San Pedro deposits are thought by the writer to be of Pleistocene 

 origin, for several reasons. First, we have a fauna with a low jDercentage of extinct 

 species. (The high percentage of the list given above is due to the large number of 

 new species and varieties, many of which are close to living forms and are probably 

 living, but which have not been reported.) The large number of species found in this 

 fauna which have never been found in beds of known Pliocene origin, and the lack 

 of several of the typical Pliocene species from its fauna, offer two of the strongest 

 arguments in favor of the Pleistocene age of the lower San Pedro series. The state 

 of preservation of the fossils is also an item in favor of their comparatively recent de- 

 position. It is worthy of note that in several cases the original coloration of the shells 

 of lower San Pedro fossils is still preserved. The vsands, too, of this formation are 

 always much less oxidized than those of the underlying Pliocene. An unconformity 

 between this formation and the Pliocene also suggests a lapse of time between 

 the two. 



