119 



through the channel between Tiburon Island and the mainland, lose their 

 strength in the center of the distinct area confirmed by the matrix south 

 of Tiburon, and thus may begin to drop their load of silt and organic 

 material which become inter-bedded with the sand. Examination of the 

 prodissoconchs of the characteristic pelecypods from this environment 

 indicated that the majority of species have a pelagic larval development. 



The second association within the intermediate shelf environment was 

 found in the southern Gulf. As in the northern region, nearly half (1 1 of 23) 

 of the index species from the matrix-associated stations proved to be 

 crustaceans. The principal environmental difference between the northern 

 and southern associations is that the majority of the 1 1 stations were 

 located on clayey bottom. Temperatures are much higher and have a 

 smaller range, thus permitting a larger number of tropical species to 

 survive there. Waters are also clearer and plankton blooms less common. 



As in the northern region, the southern portion of the intermediate shelf 

 is characterized by a fauna with a strong resemblance to that found on the 

 intermediate shelf of the Gulf of Mexico (Parker, 1960, pp. 322-23). The 

 major difference between the two Gulfs is the fact that the genera con- 

 fined to clay bottom in the Gulf of Mexico are also found on sand bottom 

 in the southern portion of the Gulf of California. Many of the species on 

 the intermediate shelf of the Mazatlan region are also twins of the Gulf 

 of Mexico species found in similar ecological conditions. In fact, the 

 number of twin species is even greater in the southern than in the northern 

 Gulf of California. This is not surprising, since the ecological conditions 

 in the San Bias to Mazatlan region more closely resemble those of the 

 northwestern Gulf of Mexico shelf than those of the Tiburon region. 



A survey of feeding types of the moUusks collected alive from the two 

 southern stations taken on sand bottom showed a predominance of pre- 

 dators and suspension feeders. Of the 61 species of mollusks taken alive at 

 these two stations, 34 are most likely predators, 15 are suspension feeders, 

 6 are probably deposit feeders, 4 algae feeders and 2 may be scavengers. 

 The feeding-type composition of the species is more normal for a sand 

 bottom than that found on equivalent sand bottoms in the northern region. 

 Eleven stations and 37 species of living mollusks were taken on clayey 

 bottoms in the southern Gulf. A breakdown of probable feeding types for 

 this assemblage produced: 12 suspension feeders, 17 predators, 6 possible 

 deposit feeders, and 2 probable algae feeders. The majority of the crusta- 

 ceans can also be considered as scavengers and predators. It was observed 

 that the species with the greatest number of individuals were the deposit 

 feeders. The large number of predators are accounted for by the fact 



