52 



The majority of the above species were also collected by the present 

 author on sand beaches, although not found in the living state. Many of 

 the species, however, were taken alive at greater depths, and may be 

 more abundant in the deeper assemblages. A much larger list can be 

 obtained from Keen (1958), although it is nearly impossible to tell which 

 mollusks of Keen's list are confined to these depths alive. A few of the 

 typical mollusks are given on Plate II. 



III. Low-salinity lagoon and mangrove assemblage. 



This environment was not studied in detail along the Pacific coast of 

 Mexico, although a few small hand collections were taken by this author 

 (Table III) and specimens were collected by other members of Scripps 

 expeditions from various brackish lagoons along the Mexican coast. This 

 discussion is confined to mollusks, but it is well known that several species 

 of commercially important shrimp and crabs occur in large numbers 

 within the lagoons between Mazatlan and Panama. Little information 

 is available as to the other kinds of invertebrates inhabiting the lagoons 

 and mangrove swamps. Inlets into the lagoons south of Mazatlan are 

 small, restricted, and are apparently closed rather frequently. For this 

 reason, few strictly marine animals will be able to survive in the lagoons, 

 and salinities are seldom high enough to support a wholly marine fauna. 

 A list of the typical mollusks found in this environment, as selected from 

 Keen (1958) is given below. Certain species in this list are quite important 

 as food, especially Anadara tuberculosa, Ostrea columbiensis and Ostrea 

 corteziensis. A few of these low-salinity mollusks are figured on Plate III 

 and all are illustrated in Keen (1958). 



Prosobranchs 

 Neritina luteofasciata 

 Neritina latissima 

 Cerithidea mazatlanica^ 

 Cerithidea montagnei^ 

 Littoridina, sp. 



Tectibranchs 

 Bulla gouldiana^ 



Pulmonates 

 Melampus olivaceous^ 

 Ellobium stagnalis 



Lamellibranchs 

 Anadara tuberculosa^ 

 Mytella falcata 

 Ostrea columbiensis 

 Ostrea corteziensis^ 

 Polymesoda mexicana^ 

 Polymesoda (7 more species known) 

 Cyrenoida panamensis 

 Mytilopsis adamsi 

 Rangia mendica^ 

 Corbula inflata 



^) Collected as a part of this study. 



