26 



pared with the distribution of the various ecological factors. A graph of 

 the depth ranges, both living and dead (Table I - Appendix) for all 

 1,150 species identified so far was devised in order to obtain an indication 

 as to where the majority of species were concentrated. The complete 

 scientific names of all species taken in this study can also be found in 

 Table 1. For this reason, describer and date of description has been omitted 

 from species names in the main body of the text. The geographic and depth 

 ranges of most of the important species found in this study were also 

 checked against the known ranges in the literature (Keen, 1958; Olsson, 

 1961 ; and various papers resulting from the Albatross expeditions). The 

 preliminary discussion of the purely biological factors which may in- 

 fluence the composition of the various assemblages resulted from examin- 

 ation of some of the original material, a survey of existing literature on 

 closely related species of invertebrates from tropical regions, and from 

 discussions with the staff of the Copenhagen University Zoological Museum 

 and Marine Biological Laboratory in Denmark. 



Description of the Region Sampled 



The majority of the biological samples collected under this Scripps 

 Institution survey were taken in the Gulf of California proper, or a region 

 which is tectonically related to the Gulf of California. This consists of the 

 Gulf of California itself, bounded by the peninsula of Baja California on 

 the west, the Colorado River delta on the north and the mainland coast 

 of Mexico to approximately Mazatlan, Sinaloa (fig. 1). It should also 

 include the Tres Marias Islands, the large irregular basins to the north and 

 south of these islands, and coast of Mexico to Bandaras Bay, since this 

 region is tectonically part of the whole complex. 



Geology and Topography. 



A description of the bordering geology and geomorphology of the Gulf of 

 California region received considerable treatment by Anderson (1950) 

 and by Byrne and Emery (1960). It is sufficient to say that, in contrast 

 to the region studied previously by the author in the Gulf of Mexico, very 

 few broad alluvial plains with abundant rainfall and river discharge are 

 present along the coast of the Gulf of California. Much of the Gulf is 

 surrounded by high mountains, with precipitous peaks descending almost 

 to the shoreline. Rocky shores are particularly abundant along the Baja 

 California coast, whereas broad, sandy beaches and mudflats are restricted 

 to the Colorado Delta area, the Costa de Hermosillo and Costa de Nayarit 



