84 



One can also see from this list that the number of species living on the 

 slope is sharply reduced from the number found on the shelf. One ex- 

 planation for this, at least in the Gulf of California, is that these depths 

 are located in the upper portion of the oxygen-minimum zone (fig. 9). 

 Difficulty in sampling on the very steep slopes characteristic of this 

 environment may also contribute to the lack of species in this collection. 

 Some of the more characteristic mollusks are figured on Plates VII and 



IX, while others are illustrated in Keen (1958). 



X. Middle continental slope, 731 to 1,799 meters. 



The middle continental slope, with its steep, dissected topography, is 

 possibly the most difficult environment in the Gulf to sample adequately. 

 Of the 12 stations taken in these depths, only four were occupied in the 

 Gulf of California. Two more samples from slope depths were taken in the 

 Sal Si Puedes Channel in the northern Gulf but were discussed under the 

 basins and troughs environment. Since so few samples were taken in the 

 Gulf proper, and the ecological conditions are so nearly uniform at these 

 depths along the Pacific coast south of San Diego, all stations taken be- 

 tween 900 and 1,800 meters along the Middle American coast were in- 

 cluded in the tabulations. These stations are: 39, 40, 84, 90, 127, 135, 

 138, 214, 215, 216, 221 and 273. Station locations and data can be found 

 in the station data list in the appendix. The devices used for these 12 

 stations were: four 1/5 m- Petersen grabs, three deep diving dredge hauls, 

 one 12-meter, one 10-meter and one 5-meter otter trawls and two rock 

 dredges. Bottom water temperature values for the 12 stations ranged 

 from 3° to 6"C., and thus could be considered lower bathyal in classi- 

 fication. Much of the middle slope region is also characterized by very low 

 oxygen concentrations of .5 to .9 ml/L (fig. 9). Only 30 species of inverte- 

 brates have been identified from these 12 stations, six of them being found 

 as dead shells only. Only three species of invertebrates were taken twice: 

 the crustaceans, Acanthephyra curtirostris and Paralomis multispina; and 

 the pelecypod, Solemya agassizi. The low oxygen concentrations and 

 inadequate sampling of these depths probably accounts for the small 

 number of invertebrates in this region. A complete list of the invertebrates 

 is given below. A few of the mollusks are illustrated on Plate VIII. 



Hexacorals Stachyptilum superbum 



Cyathocerus, sp. (dead) Swiftia, sp. (afF. S. pacifica) 



Octocorals 

 Distichoptilum, sp. nov. Monoplacophorans 



Pennatula phosphorea californica Neopilina galatheae (dead) 



