41 



in regional geology. About 100 species range no further north than Tiburon, 

 and 15 species are found only between Los Mochis and Tiburon. Twentyfive 

 species have northern limits at Los Mochis, which is also the southern 

 limit for 25 other species. The next major break in distribution is Mazatlan, 

 with 12 species having a northern limit and 49 species having southern 

 limits there. There is also an indication that San Bias may be an area with 

 slightly different temperature characteristics, since a number of species 

 have their northern limits at this point (Keen, 1958). It must be realized 

 that fig. 12 is based only on ranges of the species as found in this study 

 under variable sampling conditions. The ranges for many of these species, 

 as reported in Keen (1958), are quite different, but as it was not stated 

 whether they were taken alive throughout the reported range, these 

 data were not used. Since the sediments are radically different on this 

 portion of the shelf, no one factor can be considered decisive. At least 

 south of Mazatlan, shallow water bottom temperatures will seldom, if 

 ever, drop below 20° C, a temperature which is often considered the lower 

 limit for true tropical animals. 



The temperature distribution on the shelf and along the coast of the 

 west side of the Gulf of California is not so well known, as there is a con- 

 spicuous lack of stations where data of this nature has been recorded. 

 One can assume that inshore water temperatures at San Felipe, ranging 

 between 16° and 17°C. between maximum and minimum, will roughly 

 correspond to those at Punta Peiiasco, and are perhaps a little lower in 

 the winter. At Los Angeles Bay and along the Ballenas Channel region, the 

 annual range of temperature is much less, and maximum temperatures are 

 far lower than on the opposite side of the Gulf. They vary from about 

 14° or 15° to around 28° or 29°C., and have an annual range of 15°C. 

 These temperatures are also limiting to a number of species. Both a smaller 

 number of Panamic or tropical and a larger number of California or warm- 

 temperate species of invertebrates have been reported there. 



Water temperatures along the shores of the central portion of the Gulf 

 of California peninsula from San Lorenzo Island to La Paz range from 

 about 19° to 29° C, according to a cross-section shown in Roden and 

 Groves (1959, p. 21). The range between maximum and minimum water 

 temperature is less than at Los Angeles Bay (10°C.), but considerably 

 higher, thus permitting the existence of a much larger number of Panamic 

 species. 



Temperature records exist for the La Paz region, which is situated in 

 protected waters, and therefore are probably not representative of true 

 conditions on the outer coast of the southern portion of Baja California. 



