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"E.W. Scripps" cruise in 1941. The majority of Durham's collection con- 

 sisted of either new species (not taken alive) or were considered as fossils. 

 Even though the depths of these channels are thought of as abyssal, none 

 of the mollusk species collected in this or Durham's study were listed in 

 Clarke's (1962) tabulation of abyssal mollusks of the world. 



The problem of explaining the presence of the Californian Province 

 mollusks, cited previously for the northern Gulf basins, is a difficult one, 

 although a hypothesis is presented here. Most of these cold, shallow-water 

 species were actually collected from several shelly-sand layers in large- 

 diameter piston cores which were taken from the center of Tiburon basin. 

 Present bottom water temperatures in this basin range from 11 to 14°C. 

 These temperatures are about the same as those found during most of the 

 year on the continental shelf off California. Some of these same species 

 were also collected as sub-fossils in between 200 and 800 meters off Cape 

 San Lucas, Baja California. It is possible that these cold, shallow-water 

 species of mollusks entered the Gulf of California around Cape San Lucas 

 during the colder parts of the Pleistocene and during lowered sea level. 

 A few shallow- water (although more tropical) species of shells were dredged 

 at the edge of the shelf off San Bias, Nayarit in 110-1 15 meters (62 fathoms). 

 These shells were dated by the C-14 method by George Bien of Scripps 

 Institution. They gave ages between 17,000 and 19,000 years B.P. (Curray, 

 1962). These dates furnish some proof that sea level was at least 100 meters 

 lower than at present in the Gulf of California. If the sea level were to be 

 lowered 100 meters in the Gulf of California at the present time, there 

 would be virtually no continental shelf up to Tiburon Island. However, a 

 shelf with depths of 15 to 150 meters would exist in the northern Gulf. 

 During early Holocene times, the northern Gulf would have provided the 

 primary environment for the settlement of any California shelf species 

 which might have invaded the Gulf during lowered sea level migrating 

 slowly northward on what narrow shelf areas that may have existed. 

 Since the northern Gulf deep-water bottom temperatures are now approx- 

 imately the same as those that might have existed in shallow-water during 

 the Pleistocene, and equal to those now present on the shelf off California, 

 a number of cold-water mollusk species could survive. The fact that some 

 of these cold-water species were taken alive during this study substantiates 

 this hypothesis. The majority of the Pleistocene populations perished, 

 contributing to the shelly deposits now buried under a few feet of recently- 

 deposited clay in the northern Gulf. 



Owing to the peculiar physical conditions in the northern deep basins 



