122 



composition is therefore exactly as suspected for a deep water fine sediment 

 bottom and as found by Savilov (1961). The majority of the mollusks 

 examined from this habitat were found to have a pelagic larval develop- 

 ment, which would tend to produce a rather widely dispersed population 

 over this uniform environment. 



VII. Outer Shelf, 66 to 120 Meters, Sand Bottom, 

 Northern Gulf: 



Buchanan (1958) described an outer shelf community, designated as 

 the "offshore coarse sand community", which in terms of environmental 

 factors and faunal composition is quite similar to the Gulf of California 

 outer shelf sand bottom environment. While in the Gulf of California, the 

 sand-sized particles are mostly of terrigenous origin, Buchanan's coarse 

 sand deposits are calcareous in nature. Both sand deposits are probably 

 end results of the Holocene lowered sea level. Buchanan's characteristic 

 and abundant animals, a solitary coral, Caryophyllia, large numbers of 

 small tectibranchs, and another large Foraminifera, are either very rare 

 or not found at all in the equivalent Gulf of California environment. Some 

 of the rather scarce mollusks which Buchanan found in the coarse sand 

 community do have their equivalents in the Gulf of California at the same 

 depths. He found Eucrassatella, Phos, Cardium, two species of Corbula, 

 Nassarius, two species of Calliostoma, Pleuroliria, Pilar, Nuculana, and 

 a species of Pecten, all of which closely resemble the species of the same 

 genera found on the outer shelf of the Gulf of California. Temperatures for 

 these depths off Accra, Ghana are from 14° to 16°C. — about the same as 

 those for equal depths in the Gulf of California. Since both sediment type 

 and bottom temperature range are similar in depths of about 65 to 120 

 meters in both regions, it is not too surprising that there is such a similarity 

 of faunas as well. 



No exact counterparts of this assemblage is known from other parts of 

 the world, since uniform terrigenous sand deposits are relatively uncom- 

 mon, except where deposition has been slow for a long time, and sands 

 remain uncovered at fairly great depths. The composition of the fauna 

 in this environment is very patchy (unlike the populations on the southern 

 clayey bottoms), and few mollusk species were taken more than twice 

 out of 18 stations. The crustaceans were by far the most abundant inverte- 

 brates and many were taken at four or five stations. Unfortunately, most 

 of the crustaceans were also collected at depths both offshore and inshore 

 of the outer shelf region, and thus are poor indicator species. In many 

 ways, the Tiburon outer shelf region is a transition area rather than a 



