120 



that most of the southern stations were taken with an otter trawl, the 

 best method for collecting the more mobile animals which comprise 

 most of the predators. According to a survey of prodissoconchs by the 

 author, most of the lamellibranch species have a planktotrophic larval 

 development. Perhaps 407o of the gastropods have a pelagic stage, but 

 it is almost impossible to be sure, since the gastropod survey was based 

 on probable development from related species. Thorson (personal com- 

 munication) states that within only a very few families of prosobranchs 

 is there strict uniformity in larval development. The high percentage of 

 species with pelagic development seems reasonable, since this environ- 

 ment in the southern Gulf is one of the largest in areal extent and 

 possibly the most uniform. Planktonic larvae should therefore settle with 

 a fairly high degree of success in this region. 



VI. Outer Shelf, 66 to 120 Meters, Clay Bottom, 

 Southern Gulf: 



The animals found on the outer shelf in the Gulf of California, especially 

 on clayey bottom, are very similar to those taken in equivalent depths in 

 the Gulf of Mexico. The large number of equivalent or twin species in this 

 environment, as in the previous one, suggest that clay bottoms to depths 

 of 100 meters may also have existed across Central and South America 

 during Tertiary periods. Of the 21 typical species taken in from 70 to 120 

 meters in the Gulf of Mexico (Parker, 1960), 13 had their exact equivalents 

 in the Gulf of California in the same depths, and an additional 6 species 

 were taken at slightly different depths. It is virtually impossible to separate 

 most of these "twin" species from the two gulfs without identifying labels. 

 One is also struck by the amazing similarity of trawl and dredge hauls 

 from the clayey bottoms of the two Gulfs. The twin species constitute the 

 largest populations, so that a preliminary appraisal of unlabelled samples 

 would give very little indication of which Gulf the samples originated 

 from. To demonstrate the close similarities of this assemblage in both gulfs, 

 a list of species from both areas is appended. 



Gulf of Mexico (Parker, 1960, Gulf of California (Tables in 



pp. 323^) 75-120 Meters Appendix) (40-65 Fathoms) 



Pelecypods • Pelecypods 



Anadara baughmani Hertlein, 1951 Anadara mazatlanica 



Cuspidaria ornatissima (Orbigny, 1846) Cuspidaria pectinata 



Eucrassatella speciosa (A. Adams, 1852) Eucrassatella gihbosa rudis (Crassatella)^ 



Laevicardium fiski Richards, 1954 No equivalent at this depth- 



I 



