PLATE I. 



Figs. 1-10 show the transition from Surirella nyassae O. Mull, into S. malombae 0. 



Miill. 

 Figs. 11 and 12 show the transition from Surirella nyassae into S. EngleriO. Miill. 



var. const ricta O. Miill. 

 Fig. 13. S. Engleri, var. constricta. 

 Fig. 14. S. Engleri, var. constricta, gigantic form. 



All the figures are from microphotographs taken by Mr. A. Hesselbo ; x 300. 



