ill 



number of rare species taken at each station or the total number of species 

 occurring at any one station. Longhurst (1958) found no relationship 

 between the distribution of communities and depth, except for a change at 

 about 80 meters, with two communities inshore (a shelly-sand and mud 

 community), and one on the outer shelf and slope. The inshore com- 

 munities range into the estuaries and do not exactly correspond to any 

 of the Gulf of California communities. Relatively few species were given 

 in Longhurst's synopsis, but upon further appraisal of the discussions of 

 the various communities, there appears to be a much greater diversity. 

 There is no doubt, however, that both Buchanan's and Longhurst's 

 communities have a structure similar to those of Petersen, Thorson and 

 Sanders. The question then arises, how are the Ghana, Sierra Leone and 

 Persian Gulf regions different from the Gulf of Mexico and Gulf of Cali- 

 fornia, the former being characterized by a group of dominant animals, 

 and the latter, though having no dominant species, seems nevertheless to 

 have similar physical conditions? At least they are more similar to each 

 other in mean temperatures, than they are to Danish or Massachusetts 

 waters. The most important difference is probably in the extreme of 

 physical factors. 



This lack of species dominance in the American waters, may well have 

 several explanations, both geological and biological. The great diversity 

 of level-bottom species in the tropical and sub-tropical Americas has not 

 been confined to Recent geological times. Dall (1890b- 1903) described 

 a large number of molluscan species from what is at present called the 

 Pliocene of Florida, and 312 species of mollusks from a small horizon in 

 the Miocene of Tampa, Florida (Dall, 1915). Similar very large lists of 

 mollusk species have been described from other Tertiary deposits in 

 North and South America (Gardner, 1926-1947; Woodring, 1925, 1928, 

 1957 and 1959; and Olsson, 1922). It is difficult to determine from fossil 

 deposits how many species may have been living at any one time in one 

 spot, but it cannot be denied that there was as much diversity in the 

 molluscan fauna during the middle and late Tertiary as there is today 

 in the warmer waters of the Americas. On the other hand, a check of the 

 molluscan fauna of the Middle Atlantic Miocene (Dall, 1904) or Danish 

 Miocene (Sorgenfrei, 1958), which were warm temperate to Boreal in 

 climate, shows a very small number of species as compared to tropical 

 Miocene formations. 



Although there was a great diversity of marine mollusks in the northern 

 Gulf of Mexico (over 1,000 species on level-bottoms), the diversity is far 

 greater in the Gulf of California, where at least 1 ,400 level-bottom species 



