America, and possibly contributes to the great diversity of species in 

 some groups. 



Tiie distribution of shell remains in the sediments of the Gulf of Cali- 

 fornia gave indications as to the past history of the Gulf. Presence of 

 shallow-water species of shells in certain preferred depths indicated that 

 formerly, the sea level stood much lower than at the present time. Dates 

 for some of these shallow-water assemblages at the edge of the shelf, in 

 about 110 to 115 meters, revealed that the ages ranged from 17,000 to 

 19,000 years B.P. Shells of typical California province (colder) shelf 

 species were found in great abundance in the deeper sediments of the 

 northern Gulf basins and troughs. Some of these were also found in deep 

 waters at Cape San Lucas. This implies that during the colder Pleistocene 

 and lowered sea level, many species of mollusks were able to migrate 

 almost 700 miles to the south and up into the Gulf of California. A few 

 of these species were taken alive, but only in the deep areas of the northern 

 Gulf. Here the hydrographic conditions are similar to those now found 

 off California in shallower depths, since intense tidal mixing brings about 

 virtual isothermal and isohaline conditions from surface to bottom, with 

 stable temperatures of between 12° and 15°C. 



Qualitative comparisons were made between macro-invertebrate assem- 

 blages as found in the Gulf of California and Gulf of Mexico, as well as 

 some counterparts in other parts of the world. The characteristic species 

 of the Gulf of California environments appear to have exact counterparts, 

 usually at the subgeneric level, in similar environments throughout the 

 tropics and sub-tropics of the world. 



Introduction 



The search for new means of recognizing ancient environments of 

 deposition has utilized many methods and scientific disciplines. It has 

 usually been assumed that the most successful way of attacking this 

 problem is to thoroughly understand and recognize the various modern 

 environments and the factors which characterize them. With this concept 

 in mind, a long-term study of modern sediments was instigated by the 

 American Petroleum Institute, under Scripps Institution of Oceanography 

 in 1951. The results of this study are well-known to many, being a study 

 of nearsbore depositional environments in the Gulf of Mexico. The author 

 spent some eight years in the study of the distribution of the larger or 

 macro-invertebrates, living in or on the bottom, using them as primary 

 indicators of Recent environments. These results have been published over 



