116 



per square meter (Parker, 1960, p. 326). Five Van Veen (1/20 m^) samples 

 were also taken at the same depths in clayey sediments in the Gulf of 

 California. They averaged 6.3 individuals per sample, or 126 animals per 

 square meter, a much higher population than for the equivalent environ- 

 ment in the Gulf of Mexico. This may not be a valid comparison, as the 

 total area sampled this way was so small. These higher numbers might be 

 representative for the Gulf of California, since this zone is very narrow, 

 receiving a steady influx of animals from inshore and offshore. In the Gulf 

 of Mexico this zone is 10 to 20 miles wide, receiving little recruitment from 

 the other zones, at least in the central portion. 



Buchanan (1958) also found a benthic community in more or less 

 equivalent depths of from 8 to 20 fathoms (15 to 36 meters) off Ghana, 

 which he called the "sandy-silt community". His community bears little 

 resemblance to the association of animals found in this study, except for 

 the presence of Penaeus duoarum, one of the dominant shrimp in the 

 inshore shrimp grounds (Hildebrand, 1954) on the coast of the northern 

 Gulf of Mexico. He also found very large populations of living and dead 

 Turritella annulata Kiener. This genus did not appear in abundance alive, 

 either in the Gulf of Mexico or Gulf of California. The dominant mollusks 

 of Buchanan's sandy-silt community were also deposit feeders, although 

 the dominant gastropod is a herbivore, which was said to feed on the algae 

 growing on the Turritella shells. 



As indicated by a prolific and indicative fauna, the waters in depths 

 ranging from 39 to 65 meters constitute the most distinct portion of this 

 environment. However, many of the species found there still range into 

 shallower water, and dead shells can often be found along the shore. In 

 the previous description of the assemblages, the fauna of this zone was 

 divided into a northern and southern portion. The northern assemblage 

 was taken on sand bottom, while the southern assemblage is more con- 

 fined to clayey bottom. The northern assemblage, as revealed by the 

 computor associated stations was found exclusively on sand bottom re- 

 sulting from reworked old delta deposits presumably laid down during 

 lowered sea level and more pluvial periods (van Andel, etal., in press). 

 This assemblage and its equivalent on sand bottom to the south of San 

 Bias are remarkably similar to that found on relict delta sand deposits 

 off the Texas coast in 36 to 63 meters (20 to 35 fathoms) (Parker, 1960, 

 pp. 322-23). Again many of the mollusks from this environment are re- 

 presented by twin species from both oceans, which probably had common 

 origins in the Miocene and Pliocene shallow seas across Middle America. 

 A few of the Gulf of Mexico species of mollusks and their equivalents from 

 the Gulf of California intermediate shelf are listed below. 



