Peck u. Harrington, Plankton des Puget-Sound. 519 



slower than in the diatoms of the preceding illustration. In the three 

 types now discussed, Peridinium, Concinodiscus and Melosira, the same 

 causes may be operating 'to produce the differences in their distribu- 

 tion, i. e., irregular periods of growth and settling of dead tests where 

 such are resistent enough, and much of the vertical distribution here 

 in question 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 occupants of such water but also the indefinite accumu- 

 lations of sediment from the upper water in which so much debris of 

 organic origin is formed. 



In illustration of the action of sedimentation alone there is intro- 

 duced in the fourth column of Fig. 2 a record of the distribution of 

 certain sponge spicula, which of themselves, of course, have no power 

 of independent movement or growth and yet 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 showing 

 the sponge spicula a slight falling off in the numbers at the one- 

 quarter depth as in Melosira, and at the bottom as in Coscinodiscus, 

 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 material, 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 striaght bi-radiate type of some calcareous sponge 

 not identified by us. The important place filled in all planktonic 

 studies by the Copepoda is not vacant here since they are abundant 

 at surface, one half as numerous at ths one quarter depth (28 fathoms), 

 and rare at the middepth, 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 fore- 

 going, account was taken of many other similar forms that occur 

 less regularly and abundantly, such as diatoms Foraminifera, Infu- 

 soria etc. Among these also may be mentioned certain other objects 

 of organic origin but which are not properly to be classed as marine. 

 Thus the detritus in suspension in this water includes many bits of 

 epidermis of plants, many of the epidermal stellate hairs like those 

 upon many forms. There is also the almost constant presence of wing 

 scales from Lepidoptera, and back of all the blackish or brownish 



