144: University of California Publications in Geology [Vol. 9 



the time that certain sandstones were being deposited, appearing again 

 in the shales above. 



Strata that yield a fauna possessing distinctive characters due to 

 evolutionary change rather than to environmental difference may be 

 designated a faunal zone. The distinctive character of the zone is due, 

 then, primarily to the species whose vertical range is restricted to 

 that zone, and secondarily to those species which having reached their 

 height of development are often more abundant than at any other 

 period in the life of the species. The zone may thus be said to possess 

 restricted and characteristic species, besides many unimportant long- 

 range forms. 



The fossiliferous Cretaceous rocks of the Santa Ana Mountains are 

 divisible into three zones, each of which will be designated by the name 

 of a representative species. 



The lowermost zone of the Cretaceous of this southern region is 

 named the Actaeonella oviformis zone from the presence of a very 

 characteristic gastropod. The fauna associated with this species occurs 

 in the basal conglomerate and sandstone, which lie below the "lower" 

 shales, representing about three hundred and fifty feet of sediments. 

 The fauna obtained from these beds is based upon collection made at 

 the following University of California localities : 2130. 2131, 2134, 2139, 

 2140, 2141, 2142. 2143, 2191. 



This zone is characterized by the abundance of Actaeonella ovi- 

 formis, which appears to be restricted to the zone and by Pecten oper- 

 culiformis, Pecten calif or nica, Tricjonoarca, n. sp. c and Astartc, n. sp. a 

 which are occasionally collected within the higher zones although they 

 are nowhere so abundant as within these lower beds (see Table 1). 



The faunal zone above the Actaeonella oviformis zone is named 

 after its most characteristic species, Turritella pescaeleroensis Arnold. 

 The strata through which this fauna ranges include about eleven 

 hundred feet of shale, sandstone and conglomerate. 



This zone includes two faunas which are distinguished mainly by 

 characters due to differences in bathymetric conditions. The shales 

 just above the basal conglomerate and sandstone yield a number of 

 gastropods and cephalopods not found abundantly in the coarser sedi- 

 ments of the underlying or overlying strata. This deeper-water fauna 

 appears to be replaced by a fauna of shallower water characterized 

 by the abundance of such species as Turritella pescaeleroensis, Crass- 

 atellites lomana, and C. conradiana var. tuscana, but it again appears 

 in a slightly modified form in certain higher shale beds. This deeper- 



