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



face, have been much wider, and would in fact have been more nearly 

 like that represented for the Peridinium of column I. If now they 

 were to die suddenly out at the surface and the debris were to gra- 

 dually settle into deeper layers an irregularity would there be caused 

 such as is here evident at the three-quarters depth. Such irregular 

 periods of growth are doubtless less common in the deeper layers of 

 water since the conditions must be more stable in these depths, and 

 this striking increase of Coscinodiscus at the 84 fathom depth, there- 

 fore, probably represents a previous period of active growth in the 

 upper strata, and if one were to restore to life and the former con- 

 ditions of growth all the dead and crushed debris of the three-quarters 

 level here brought out it would doubtless form such a column as is 

 given in the one just described for Peridinium. And conversely if 

 conditions of active reproduction were suddenly to become unfavorable 

 at the surface for the two species of Peridinium here plotted (column I), 

 and they were provided with a resistent test like that of the large 

 diatom, their vertical distribution would tend to assume, by cessation 

 of growth and settling of debris, that which has now been described 

 for Coscinodiscus. 



The dividing line between the two type species /. e., between 

 the quantity of Coscinodiscus occulus iridis and that of the large and 

 beautiful C. asteromphalus in this second column is shown for each 

 level by a small white vertical bar placed in the black areas. All 

 to the left of these bars is C. occulus iridis (with a few individuals 

 of similar size from other species or genera), while all to the right 

 of these white bars is C. asteromphalus. 



In the third column of Fig. 2 portraying the distribution of the 

 diatom Melosira, is shown a very uniform increase in the numbers of 

 that organism from surface to bottom with a slight falling off at the 

 one-quarter depth. The individuals of this species here enumerated 

 were laid in short chains and each short chain was counted as a unit 

 in the determination, but they are so small and transparent that it 

 did not seen advisable to attempt the separation of those living at 

 the time of the collection from the dead, and the whole, therefore, is 

 plotted as living material. It is certain, however, that the diatom 

 can live at this depth as many healthy chains can be found at the 

 bottom stratum and we here conclude also that under other conditions 

 the abundance of this form would be much greater in the overlying 

 strata of water, and that the more rapid change of conditions above 

 has left a larger quota alive in the slower-changing depths. That is 

 to say the increase is not so much due to conditions of active growth 

 below as to the disappearance of a large proportion of those in the 

 superficial layers, together with a settling of the debris into the levels 

 below although this latter process in this delicate form must be much 



