76 



VI. Outer shelf, 66 to 120 meters, clay bottom, southern Gulf. 

 A portion of the outer shelf environment in the Gulf of California from 



Los Mochis to just south of Mazatlan was verified by the contingency 

 matrix as being a distinct region. Six stations proved to be closely associ- 

 ated by five commonly occurring species, which were abundant enough in 

 numbers per station to identify the environment. The number of species 

 seem to decrease at these greater depths and especially in the matrix-desig- 

 nated region which is based on eight stations taken in a silty clay bottom 

 (fig. 25). The eight stations were sampled by six kinds of devices (Table III 

 and fig. 2a), yet each device produced essentially the same species attesting 

 to the uniformity of the assemblage. The devices used were: one orange 

 peel grab, two shell dredges, two 12-meter and one 3-meter otter trawls, 

 one rock dredge and one box dredge. The five associated species were: 

 a polychaete, Protula siiperba, two gastropods, Conus arcuatus and Cruci- 

 bulum, sp. (allied to C. striatum of the Atlantic), and two lamellibranchs, 

 Chione kellettii and Anadara mazatlanica. 



A complete list of the living species (with the most abundant in number 

 starred) from the outer shelf clay bottom environment can be found in 

 Table II of the appendix, while all species living and dead from these 

 stations can be ascertained from a study of Table I. Two of the outer 

 shelf stations in the southern Gulf were taken on sand bottom, but only 

 two species (both epifaunal in nature) from these stations were also found 

 on clay bottom. The more important mollusks are illustrated on Plate VI. 



VII. Outer shelf, 66 to 126 meters, sand bottom, northern Gulf. 

 All stations in depths of from 66 to 126 meters from just south of 



Tiburon to Punta Peiiasco in the northern Gulf were taken on sand 

 bottom. This assemblage was completely different from the one found on 

 clay bottom in similar depths to the south. These stations (fig. 25) occurred 

 on what appears to be a residual sand bottom resulting from eustatically 

 lowered sea level, since nearly every station had a large number of dead 

 mollusk shells of species usually found living close to shore. A total of 

 93 living species of invertebrates, mostly epifaunal, were collected from 

 18 stations, giving an average of only five species per station. Owing to 

 the heterogeneous composition of the 18 stations on sand bottom in the 

 north, no closely associated group of species resulted from the contingency 

 matrix. Only two species of invertebrates occurred at enough stations in 

 this environment to be considered abundant (in the sense used by Re- 

 mane, 1940): the echinoid, Clypeaster eiiropacificus and the pelecypod, 



