78 



Lucinoma annulata, which are also found in deeper waters in the northern 

 Gulf. A large number of pelecypods, found as dead shells, were con- 

 sistently found in these and only these stations, but because of the pre- 

 sence of so many dead shallow-water species, one cannot be certain that 

 they lived at these depths. The most interesting aspect of this assemblage 

 was the large number of living gastropod and crustacean species. Many 

 of these species also occurred on inshore sand bottom to depths of less 

 than 20 meters. Apparently, the nature of the bottom is more important 

 than depth in the northern Gulf, so far as limiting the distribution of the 

 more mobile invertebrates is concerned. One likely reason for depth being 

 non-restrictive is that due to intense tidal action, the waters are thoroughly 

 mixed from top to bottom. Although almost half of the samples were 

 collected by shell dredge, five other devices were used (Table III), which 

 may have also contributed to the heterogeneity of the assemblage. These 

 samples were: seven shell dredges, one 10-meter and three 3-meter otter 

 trawls, five rock dredges, one orange peel grab, and one 1/10 m^ Peter- 

 sen grab. 



All invertebrates taken in the northern Gulf outer shelf environment, 

 living and dead, can be determined from Table I, while living species only, 

 with the most abundant forms starred are given in Table II in the appendix. 

 A few of the common moUusks are figured on Plate VI. 



VIII. Northern Gulf basins and troughs, 230 to 1,500 meters. 



Owing to the peculiar temperature and sediment conditions in this deep 

 portion of the northern Gulf of California, this environment may well be 

 unique in the world today. The great tidal exchange through these basins 

 and channels create virtually uniform temperature, salinity and oxygen 

 conditions from about 30 to 50 meters close to the surface down to the 

 bottom at 1,500 meters. This turbulence plus the geological background 

 of the areas created an almost uniform sand to gravel bottom in all depths 

 (fig. 7). Since two very important ecological factors (water temperature 

 and sediment size) which are apt to bring about vertical stratification of 

 fauna are eliminated, it is interesting to see what effect, if any, the in- 

 fluence of pressure has had on the composition of the fauna in the deeper 

 portions. 



The species found in this environment must be separated into two parts, 

 one composed of the living species and another which is composed mostly 

 of shell remains. Normally, dead shells would not be considered too 

 important, but in this instance it is interesting enough to be dealt with 



