384 TRANSACTIONS OF THE [dEC. 13, 



the levels below, although this latter process in this delicate 

 form must be much slower than in the diatoms of the preceding 

 illustration. In the three tj'pes now discussed, Peridinium^Cos- 

 cinodiscus and Ilelosira, the same causes may be operating to 

 produce the differences in their distribution, i. e., irregular per- 

 iods of growth and settling of dead tests where such are resist- 

 ant enough, and much of the vertical distribution here in ques- 

 tion is due to a greater or less extent to the specific gravity of 

 the various elements. One encounters at the lower levels not 

 only the immediate occupant of such water, but also the indefin- 

 ite accumulations of sediment from the upper water in which so 

 much de'bris of organic origin is formed. 



In illustration of the action of sedimentation alone there is 

 introduced in the fourth column of Plate II. a record of the 

 distribution of certain sponge spicula, which of themselves, 

 of course, have no power of independent movement or growth, 

 and 3'et which are constantly found through the water, and 

 which increase quite regularly to the bottom. The source 

 of 'these is probably in the shallower waters of other localities, 

 from which they are introduced hither by tide currents near 

 the surface, and thus sown, as it were, through the whole depth 

 of the channel. There will also be noticed in this column show- 

 ing the sponge spicula, a slight falling off in the numbers at the 

 one-quarter depth as in Melosira, and at the bottom, as in Cosci- 

 nodiscus, and it is therefore reasonable to suppose that they are 

 obedient to the same influences in these respects, although we 

 assume that the greater part of this very regular distribution of 

 the sponge spicula is due to the gradual accumulation of mate- 

 rial, for long periods, by sedimentation from the whole volume 

 of water in which they are mixed by currents and other modes 

 of diffusion. The spicules here recognized are of the straight 

 bi-radiate type of some calcareous sponge not identified by us. 

 The important place filled in all planktonic studies by the Cope- 

 poda is not vacant here since they are abundant at surface, one- 

 half as numerous at the one-quarter depth (28 fathoms), and rare 

 at the mid-depth, only a single individual having been found 

 there in one of the analyses of that level. There were also Nauplii 

 of this form in association with the adult stages, one Nauplius 

 on an average to two adult individuals at each level. 



Besides these organisms thus plotted and described in the 

 foregoing, account was taken of the many other similar forms 

 that occur less regularly and abundantly, such as diatoms, Fora- 

 minifera, Infusoria, etc. Among these also may be mentioned 

 certain other objects of organic origin but which are not prop- 

 erly to be classed as marine. Thus the detritus in suspension in 



