172 



Life 



800 





Figure 149. Graph showing percentage of 316 samples in various depth ranges which contain one or more specimens 

 of sihceous sponge, of anemone, and of other kinds of animals. At right is depth distribution of average biomass 

 (exclusive of algal bottoms), average number of species, and average bioindex of samples from all areas except the 

 impoverished bottoms of San Pedro and Santa Monica Basins. Data from Hartman (1955a, 1956) and Hartman and 

 Barnard (1957, 1958, and unpublished). 



quent at great depth, pelecypods at shallow 

 depths, and pelecypods and gastropods 

 nearer the water surface. The absence of 

 mollusks on the lower slopes and floor of 

 San Pedro Basin was also noted by Natland 

 (1957), who with Alex Clark studied small 

 samples collected in 1932. Natland pointed 

 out the fact that mollusks are seldom en- 

 countered in the shales of the now-filled 

 Los Angeles and Ventura Basins; this absence 

 may mean simply that these two basins were 

 formerly similar to the San Pedro and Santa 

 Monica Basins in having their sills within 

 the depth range of the oxygen minimum of 

 the open sea and thus containing bottom 



water inhospitable for mollusks, although 

 not so for foraminiferans. Had their sills 

 been deeper, these ancient basins might be 

 expected to contain fossil mollusks like those 

 now living in the deeper-silled Santa Cata- 

 lina and other outer basins. 



Although few of the animals from basin 

 floors have hard parts capable of preserva- 

 tion, it is possible that evidence of their 

 burrowing activities may remain in the 

 geological record of the sediments as inter- 

 ruptions of bedding planes (Schafer, 1956). 

 Most photographs of the basin floors (Figs. 

 151, 152) show a hummocky microtopog- 

 raphy made by hillocks about 15 cm high 



