100 



uniformity of this assemblage over long stretches of coast line, even where 

 there are considerable portions of rocky coast separating the beaches. 



This environment provides the most useful tools for the interpretation 

 of the Recent geological history of any continental shelf. Many sturdy 

 mollusk species are confined to this depth, and are thus excellent indicators 

 of sea level. Presence of members of this assemblage as dead shell on deeper 

 parts of any continental shelf is usually an indication of previously lowered 

 sea level unless transported by ice. The presence of submerged shorelines 

 resulting from the last glaciation was substantiated and dated in the Gulf 

 of Mexico by using typical mollusk shells of intertidal and nearshore shelf 

 species (Curray, 1960 and Parker, 1960). Time and again, many of the 

 Gulf of California species typical of this environment appeared as dead 

 shell on portions of the middle and outer continental shelf off San Bias 

 and Hermosillo. They were almost always associated with sand deposits 

 at preferred depths or on slight topographic highs, which could have 

 resulted from a once lowered sea level (van Andel, etai, in press). Since 

 many of the mollusk species in this habitat are subjected to the pounding 

 of the surf, most of them are quite sturdy and resistant to abrasion. They 

 are, therefore, the most likely species to remain in place and undamaged 

 over long periods of geological time. This observation is substantiated by 

 their abundance in Tertiary fossil localities. Lists of mollusks from un- 

 consolidated sand and sandstone deposits from the Gulf of California, 

 compiled by Emerson (1960a) and Hertlein and Emerson (1959), and 

 presumed to be of Pleistocene age, are dominated by the sand-flat species, 

 most of which were in a good state of preservation. 



111. Low Salinity Lagoons and Mangrove Swamps: 



This assemblage, with its abundant populations of oysters, Rangia, 

 Mytilids, Corbiculids and Penaeid shrimp, closely resembles the low- 

 salinity, river-influenced assemblage of the northern Gulf of Mexico coasts 

 (Parker, 1959). The presence of the immense Arcid, Anadara tuberculosa, 

 the large Corbulas, and many more species of corbiculids in the Gulf of 

 California lagoons, constitutes the principal difference between the Gulf 

 of Mexico and Gulf of California faunas. Otherwise, 11 of the 19 listed 

 species for the Gulf of California lagoons have their exact counterparts in 

 the Gulf of Mexico. Many of the same genera and sub-genera of mollusks 

 frequenting these low-salinity areas are also known from similar regions 

 in South America, Africa and tropical, sub-humid Asia. 



It is difficult to determine the dominant feeding type or usual form of 

 larval development for the low-salinity lagoon assemblage, as so little is 



