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temperatures of 1° to 2°C., but in many places depths of much less than 

 2,000 meters. If depth or pressure were the determining factor, then 

 animals normally found in the middle continental slope environment depths 

 should also occur at similar depths in the northern basins of the Gulf of 

 California. On the contrary, a fauna of much shallower water but known 

 previously from equivalent bottom temperatures was found. If temper- 

 ature is the primary limiting factor, why do many of the Arctic and 

 Antarctic bottom faunas remain mostly near the poles? Relatively few 

 Arctic species are ever found on the abyssal sea bottom, south of 30° 

 north latitude, nor are Antarctic species found north of 30' south latitude, 

 even though bottom temperatures typical of shallow polar waters extend 

 over most of the abyssal sea floor. 



No community or ecological studies concerned primarily with this 

 particular environment are known from the literature. Numerous col- 

 lections have been taken on the slope and many species have been de- 

 scribed from these depths, but as in the upper slope environment, a 

 synthesis of this material is difficult. The best descriptions of partial 

 assemblages of the middle continental slope environment can be found in 

 the narratives of the Challenger (Murray, 1895, and Moseley, 1880), 

 Albatross (Townsend, 1916), Siboga (Weber, 1902), Swedish 

 Albatross (Nybelin, 1951), and Galathea (Bruun, Greve and Sparck, 

 1956, and Madsen, 1961) Expeditions. 



All but five of the mollusk species collected alive from this environment 

 are deposit feeders. The four octocorals may be considered suspension 

 feeders, although the term predator might be a better classification. No 

 attempt was made to classify the crustaceans as to feeding types, although 

 the shrimp living just off the bottom and the large decapod crabs are 

 probably predators or scavengers. The fact that some suspension feeders 

 exist at these relatively great depths is not too surprising, since surface 

 production is still rather high over this portion of the slope, at least along 

 the Middle American coast. No estimate can be given of the larval develop- 

 ment of the mollusks taken in this environment. Natland (1933) described 

 a Pliocene deep sea fauna from southern California Tertiary basins. A few 

 mollusk species related to those occurring in this assemblage have been 

 found in Miocene and Pliocene formations of the West Coast of North 

 America (Grant and Gale, 1931), and the Pliocene of Costa Rica and 

 Panama (Olsson, 1942), so that this assemblage may still have some use 

 in paleoecological interpretations, even though this environment is con- 

 sidered too deep for most geological formations containing abundant 

 fossil material. 



