123 



distinct entity. The breakdown of the thermocline in this environment 

 because of intense turbulence and upwelling may contribute substantially 

 to the transitional nature of this assemblage. The majority of pelecypod 

 species are suspension feeders, while 80% of the living gastropods are 

 predators. A few of the gastropods species and about half of the lamelli- 

 branchs seem to have a pelagic larval development, which again is tran- 

 sitional in larval development types between shallow and deep waters. 



VIII. Northern Gulf Basins and Troughs, 230 to 1,500 Meters: 

 No detailed studies have previously been carried out in any other region 

 with these peculiar environmental characteristics. Somewhat similar 

 environments may exist elsewhere, for instance the Red Sea, with its great 

 depths and warm bottom temperatures (21 to 23° C.) and high bottom 

 oxygen. However, apart from Fuchs' (1901) discussion of the "Pola" 

 expedition, the present author knows of no comprehensive study of the 

 fauna of the deeper parts of the Red Sea. Surprisingly, the benthic fauna 

 from 300 to 2,190 meters in the Red Sea was composed of typical bathyal 

 species resembling those found in the southern Gulf of California. Since 

 bottom temperatures are so high in the deep parts of the Red Sea, there 

 seems to be little reason for vertical stratification of animals. The lack of 

 at least some shallow-water species at these depths is also puzzling. One 

 also wonders how these typical bathyal species of invertebrates and fish 

 were able to cross the shallow entrance to the Red Sea from the Indian 

 Ocean. This may be one instance of pressure being a limiting factor to 

 distribution. Baldi (1961) has described a Miocene fauna from Hungary 

 which he presumes had lived in an environment similar to that found in 

 the Gulf of California northern basins and troughs. A few of the genera 

 reported by Baldi, such as Surcula, ''Gemmula'\ Amusium, a giant Den- 

 talium and Bathytoma ( ?) do occur at bathyal depths elsewhere and appear 

 to superficially resemble those mollusks taken at bathyal depths in the 

 Gulf of California. It may be hazardous, however, to state that Baldi's 

 Nassa-Pleurotoma clay represents the same environment as the northern 

 Gulf basins. More likely equivalents may appear in the Southern California 

 and Venezuelan Tertiary basins. 



It is significant that none of the living species of mollusks from the 

 northern basins, and especially those from the 1,500 meter deep channels, 

 have ever been found as part of the lower bathyal or abyssal fauna else- 

 where. None of these species can be considered deep water species, the 

 majority having been found previously in shelf depths. Additional species 

 of mollusks were taken from the deep channels by Durham (1950) on the 



