BY G. I. PLAYFAIK. 541 



tioii, perhaps, or a sudden rise of temperature. Cells also 

 were noted, in which it was quite clear that the ridges of the 

 membrane are merely rows of granules coalesced and accen- 

 tuated by incrassation (PI. Iv., f. 4/). Further, although 

 most specimens present individual peculiarities and irregu- 

 larities, such as might reasonably be expected in growing 

 cells, the general disposition of the inter-tabular grooves or 

 ridges (when present) is the same, front and back, for every 

 type found in the plankton. 



This arrangement of the inter-tabular grooves is the same as 

 figured for P. tahulatum by Klebs, Organ, einig. Flagellaten- 

 Gruppen, T.ii., f.28; and by Apstein, Siisswas.serplankton, p. 152, 

 f.52, the figure in the latter being taken from Stein's Organismus, 

 T.xi., f.ll. It is, therefore, impossible to doubt that all the 

 types observed in the plankton are stages of development in the 

 life-history of a form which is mei'ely a local variant of P. tabu- 

 latum. Corroborative of this, is the fact that cells were noted 

 with the usual reticulate membrane of that type. 



I consider it, however, as quite certain that all the various 

 freshwater "species " of Peridinium are polymorphic forms of one 

 true species. Schilling's diagrams in Siisswasser Peridineen, T.iii., 

 figs.21-25, show that the arrangement of the plates in the hypo- 

 valve is the same for P. tahulatum, P. cinctum, P. bipes, P. quad- 

 ridens, and P. iimbonatum (compare also our Pl.lv., f.ll), and 

 the difference in their epivalves is very slight. More significant 

 still is the fact that, although Peridinium propagates largely by 

 micro-zoospores, only a single resting cell has been noted(Penard, 

 Peridiuiacees du Leman, PI. iv., f.4), and this is connected (as is 

 natural) with the most minute form of Peridinium known. The 

 same resting cell, I have obtained from swampy ground in Auburn 

 in some quantity, and, associated with it, transition-forms show- 

 ing plainly its development into a Peridinium — a local variant of 

 P. inconspicuum Lemm. Were the various " species " really 

 distinct, there would be a distinct resting cell and life-history for 

 each; but as it is, when any local "species" is investigated 

 thoroughly, a series of forms is brought to light always leading 

 down to P. pusillum Penard, or to one of its variants such as 



