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large body of data for a number of ecological factors has enabled the 

 author to explain some of the vagories of faunal distribution within this 

 interesting region, based both on the distribution of the abundant animals 

 and the boundaries of the physical-chemical factors. The biology (develop- 

 ment and feeding habits) of some of the components of these assemblages 

 has been superiicialiy investigated, offering a few more parameters to help 

 explain marine invertebrate distribution. Finally, various events in the 

 past (the geological factors) have afforded yet a few more explanations 

 for the "whys" of animal zoogeography. 



In all, 12 distinct assemblages of benthic invertebrates, based on the 

 close association of the most numerous and characteristic species, and also 

 representing 12 environments or major habitats, were found in the area 

 included in this study. This region encompassed the west American coast 

 from San Francisco, California to Guatemala, and included the entire 

 Gulf of California. These 12 assemblages have a few abundant species but 

 were generally characterized by a great diversity of species and relatively 

 few individuals of any one species. The 12 environments which have 

 distinctive assemblages of invertebrates are: 



I. Intertidal rocky shores. 



II. Intertidal sand-beach and sand-flats to 10 meters. 



III. Low-salinity lagoon and mangrove mud flats. 



IV. Nearshore, sand and sand-mud bottom, 11 to 26 meters. 

 V. Intermediate shelf, 27 to 65 meters. 



VI. Outer shelf, 66 to 120 meters, clay bottom, southern Gulf. 



VII. Outer shelf, 66 to 120 meters, sand bottom, northern Gulf. 



VIII. Northern Gulf basins and troughs, 230 to 1,500 meters. 



IX. Upper slope, central and southern Gulf, 121 to 730 meters. 



X. Middle continental slope, 731 to 1,799 meters. 



XI. Abyssal southern borderland basins and outer continental slope, 



1,800 to 4,122 meters (limits of sampling depths). 



XII. California borderland basins, 1,641 to 2,358 meters. 



Many of these environments and their assemblages have counterparts 

 in other parts of the world, although few benthic ecological studies have 

 been carried out in tropical and sub-tropical areas of comparable richness. 

 Those environments investigated elsewhere are: the intertidal rocky 

 shore; intertidal sand flats; low-salinity lagoons; nearshore shelf, inter- 

 mediate shelf, outer shelf on clay and sand bottom, upper slope, middle 

 slope and abyssal slope. Those with no exact known counterparts in the 

 world are: the northern Gulf basins and California borderland environ- 



