102 



fore, lagoonal areas must be part of the shoreline or allied with the terres- 

 trial environment. The occurrence of estuarine shells at the shelf edge off 

 San Bias (van An del, et ai, in press) constitutes excellent proof that 

 a shoreline existed there at one time. 



Although not investigated at all in the Gulf of California, the inlet 

 environment should not be completely neglected, since the very existence 

 of lagoons and estuaries along the Gulf of California coast means that 

 inlets or access to these lagoons from the Gulf must also exist. The inlets 

 were a very distinct and characteristic habitat for the Gulf of Mexico 

 lagoons and sounds as pointed out in Parker (1955, 1956 and 1959). The 

 physical factors which set this environment apart from other lagoon or 

 open ocean areas are: first of all, two-layered, strong tidal currents which 

 generally sweep the bottom clean creating either a shell-gravel or hard 

 sand bottom, variable salinities and temperatures, generally in the upper 

 range of the climate in which they are located, and finally a continual sup- 

 ply of nutrients, both from the estuarine side and open Gulf or ocean side. 



A few observations were made in the inlet into Mazatlan harbor in 

 April, 1959. One species of mollusk, Atrina (probably mama), certainly 

 must have been abundant, as native divers were harvesting them in large 

 numbers and have been doing so for years. This has proved to be one of 

 the index mollusks for the inlet environment on the Gulf of Mexico coast 

 also (Parker, 1956 and 1959). Bottom samples were difficult to obtain 

 as the bottom was too hard, but a few small Van Veen samples did produce 

 a number of living Olivella, Tellina, Crepidula and small Venerids. All of 

 these genera were found to be very characteristic in the Gulf of Mexican 

 inlets as well and seem to be a typical ISO-Tellina community. The 

 stations from this inlet are: 11, 112, 113, 114, all taken with a 1/20 m"^ 

 Van Veen grab. 



No other comparable studies of inlets as distinct habitats for macro- 

 invertebrates are known to the author, although Henning Lemche 

 (personal communication) has recognized that the conditions which 

 characterize inlet environments usually produce a rich nudibranch popu- 

 lation, as well as a rich growth of algae, bryozoans, hydroids and turbel- 

 larians. Investigations of similar environments in other parts of the world 

 produced many of the same characteristic organisms, with the exception 

 of those mollusk genera which are exclusively tropical, such as Pinna 

 and Atrina. 



IV. Nearshore Sand to Sand-Mud, 11 to 26 Meters. 



This environment corresponds closely to the shallow or nearshore shelf 

 environment off the Mississippi Delta (Parker, 1960), and portions of both 



