224 THE MEDUSAE. 



assume that this disappearance is absolute, the evidence afforded by hauls 

 one hundred and fifty miles apart being far too scanty to warrant such 

 a conclusion. On the contrary, it is hardly conceivable that some or all 

 of these species do not occasionally occur in the barren area as does Tetror- 

 chis erythrog aster ; and even with this reservation, the difference in degree, if 

 not in kind, between the gradual east-west diminution of the surface forms, 

 and this sudden disappearance of the common intermediate species, is as 

 striking as it is unexpected. 



The occurrence of the holoplanktonic surface Medusae is about what 

 might be expected in any oceanic region traversed by a well-marked ocean 

 current. Indeed a very similar diminution in number of individuals is con- 

 stantly to be seen on leaving the Gulf Stream ; but to account for the 

 conditions which obtain in the distribution of the intermediate Medusae 

 is by no means easy. 



I must confess that I have been unable to find any satisfactory expla- 

 nation for the apparent limit to their westward dispersal. This limit, I 

 might add, is more or less effective not only in the case of Medusae, but 

 of the bathypelagic fauna as a whole; for in all classes of organisms the 

 diminution, whether or not approaching total disappearance, was much 

 more sudden among the intermediate than among the surface forms. Fur- 

 thermore, comparison with chart 3" of the general report of the Expedition 

 (A. Agassiz, :06) shows that the line of disappearance of the interme- 

 diate Medu.sae coincides almost exactly with the line of southern limit 

 of good trawl hauls, so that it appears that, as might be expected, this 

 quantitative diminution is not limited to the pelagic fauna alone, but holds 

 true also for the bottom species, which depend ultimately upon the 

 pelagic fauna and flora for their food supply. Temperature, so frequently 

 a barrier to the dispersal of marine organisms, offers no satisfactory expla- 

 nation of the present case. For although the surface temperature showed 

 a very wide range from the coast westward, the extremes observed being 

 65° and 82°, below the depth to which the diurnal warming of the sun 

 may be expected to penetrate (say 50 fathoms), the temperatures were 

 remarkably uniform throughout the entire area traversed during the cruise. 

 Thus at 200 fathoms the extremes of temperature recorded are 56.7° and 

 48.5°, a range of 8.2° ; at 300 fathoms, 48.2° and 42.7°, a range of 5.5° 

 (omitting one record of 53.2°, due no doubt to some local disturbance) ; while 

 at 400 fathoms, with extremes of 42.5° and 41.9°, there was a range of 



