55 



the slight information available from the present physical data, it would 

 seem that extremes of water temperature play an important part in 

 producing a somewhat different infauna in the southern than in the 

 northern part of this environment. As shown in fig. 12, inshore water 

 temperatures range from 15^ to 30° C. in the Tiburon region, while south 

 of Mazatlan, the range is much less and higher (22° to 30° C). This is 

 somewhat substantiated in that if one compares the two lists in Table II, 

 it can be seen that there is usually a replacement of species within the 

 same genus between the two areas, and that if one looks at the published 

 ranges of these species, the southern species are those which have not been 

 previously cited in the northern Gulf. Sediments were also somewhat 

 finer and less well-sorted in the southern region. One of the reasons so few 

 grab samples were taken in the Tiburon region, was that none of these 

 sampling devices would work in the hard sand and shell-sand bottom. The 

 shell dredge was the only device that would dig down into the bottom, and 

 in fact was frequently the only source of sediment samples for the geo- 

 logists, as their coring devices would not operate either in the hard bottom. 

 The Tiburon region is also affected by upwelling (fig. 10), while no up- 

 welling takes place in the San Bias region. This factor alone creates not 

 only differing water temperatures, but also varying food supply. 



Actually, the Tiburon region in depths of four to thirty meters is very 

 complex in patterns of invertebrate distribution and no different from any 

 other marine bottom in this respect. Five different groupings of animals 

 resulted from the contingency matrix, each with a slightly different set of 

 stations in common (fig. 16). Since both living and dead occurrences were 

 used to establish the stations groupings as shown in fig. 16, these patterns 

 may not be very significant. They are given, primarily to show the com- 

 plexity of distributions which may result from such a program. Table IV 

 gives the list of the species which comprised the index groups from the 

 computer program, listed as to their importance and number of stations 

 common to each within its group. The tabulation of the known physical 

 factors for each group is also shown. 



Since the above computor analysis produced a somewhat confused and 

 probably false impression of the true assemblage (since both living and 

 dead records were used), a more reliable assemblage was devised from a 

 combination of all of the index groups, using the somewhat more subjective 

 method of analysis mentioned in the discussion of methods. All species 

 which were involved in the matrix analysis were listed and compared as 

 to common station occurrence and abundance only as living records. It 

 was then possible to select the stations where the greatest number of these 



