THE OSTEACODA, 



303 



Supposing that some oceanic species do actually oscillate vertically in the manner attributed to 

 Limnoplankton and Neritic Plankton, there seem to be two possible methods. (1) In the one case, a 

 species may oscillate (as it were) in one piece, plurimum and extremes alike falling and rising simul- 

 taneously ; in this case the distribution spindle will retain its shape and proportions but will alter its 

 position in depth. As an illustration of what would seem likely to happen in this case, the 

 accompanying diagram (fig. M) of the actual distribution of curta during the cruise has been drawn. 

 On the ordinates of the twenty-four hours, the numbers of specimens captured x time-factor have been 

 plotted on both sides of abscissae representing 25, 50, 75, and 100 fathoms *. The spindles obtained by 

 joining up the ordinates seem to show a plurimum at 25 fathoms at 8.0 p.m., at 50 fathoms at 11 p.m. 

 and again at 1.0 p.m., and at 75 fathoms about midnight. These can be interpreted to mean that the 

 plurimum at 75 fathoms at midnight rises to 50 fathoms by 1 p.m., and to 25 fathoms by 8 p.m., that it 

 sinks again to 50 fathoms by 11 p.m., and back to 75 fathoms at midnight, the fall, aided by gravity, 

 being naturally more rapid than the rise. The slight increase at 100 fathoms towards midnight may be 



y 



i^S 



25 FM 



Fig. M. 



5DFM. 



75 FM. 



IDOFM. 



MID-DAY 

 PMIO 

 2D 

 50 

 4D 

 5Q 

 BD 

 7Q 

 BD 

 9-Dl- 

 100 

 lIDh 



AMID- 

 2-0- 

 3-0 

 4-0 

 5-D 

 BD 

 7G 

 B-0 

 9D 

 ID-D 

 ll-O 



I1ID-DAY 



due to reinforcement either from above or below, but the numbers were fairly steady at this level all 

 through the twenty-four hours. When similarly treated, there are indications of the same sort of thing 

 in magna, rotundata, and the A larvte, but they are not so clear. The occurrences of the other species 

 ■were too scattered, and their numbers too few to give any clear result. This oscillation, if true, would 

 account for the puzzling two plurima at different levels, noticeable in the distribution spindles, not only 

 of Ostracods but also of some other species already treated in this series of papers. Thus, for example, 

 the double plurimum of magna (fig. L) is apparently produced by a plurimum at 50 fathoms about 

 9.0 p.m., and another at 100 fathoms about 3.0 a.m. 



(2) If only a certain percentage of the specimens of a species rise and fall, the gain and loss will 

 balance at each horizon except the highest and lowest ; the plurimum will remain steady, and the spindle 

 will alter its shape and proportions. None of the diagrams made from the ' Research' specimens seem 

 to suggest this. 



The specimens at fm. were too few to bo significant. 



