191(3] 



Packard: Cretaceous of Santa Ana Mountains 



143 



The bivalves are represented by the largest number of species as 

 well as by the largest number of individuals of all the groups enumer- 

 ated above. Trigonoarca, a genus commonly collected in the Trichi- 

 nopoli group of the Indian Cretaceous, is represented in the Santa Ana 

 Mountains by three new species. Two other genera, Liopistha and 

 Gastrochacna, have not heretofore been reported from the Cretaceous of 

 California. Members of the genera Tellina, Crassatellites, Meretrix, 

 Lima, and Pecten are frequently obtained from the Santa Ana region. 



The gastropods are most abundantly represented by members of 

 the families Turritellidae, Aporrhaidae and Volutidae. The Turritellas 

 are very abundant in this southern region, whereas in more northern 

 Upper Cretaceous localities of California and especially in Washington 

 and British Columbia they are rare. 



The cephalopods are represented in our collections by a number 

 of imperfect specimens. Two or possibly three large nautiloids. 

 resembling certain Indian species, occur in calcareous nodules within 

 the lower shales. Several ammonoids have been found within these 

 shales. Of these forms the genus Schloenbacliia is most commonly 

 obtained. 



FAUNAL ZONES 



Faunal studies of marine life carried on within the North Sea, 11 

 at Woods Hole 12 and elsewhere show intimate relationship between the 

 distribution of marine organisms and the factors of their environment. 

 The character of the bottom, or in geologic terms the type of deposition, 

 determines in a large measure the facies of a given fauna. Thus the 

 different types of bottom are characterized by different faunal associa- 

 tions. 



It seems highly probable that under conditions of littoral deposition 

 a sand-dwelling fauna would be replaced by a quite different fauna 

 if conditions of sedimentation were changed. Upon the return of 

 favorable conditions the earlier fauna might again occupy the same 

 region. This effect of the changing conditions is seen in the vertical 

 distribution of a number of large gastropods belonging to the Volutidae. 

 These forms are most abundant within the Santa Ana Mountains in 

 the "lower" shales, and apparently were forced to migrate during 



11 Peterson, C. G. Joh., Valuation of the Sea 11, Report of the Danish 

 Biological Station 21, pp. 1-44, 1915. 



12 Sumner, F. B., Osburn, B. C, Cole L. J., and Davis, B. M., A biological 

 survey of the waters of Woods Hole and vicinity, Bull. U. S. Bur. Fish., 31, 

 pp. 1-860, 1913. 



