40 



MR. C. BAILEY ON THE STRUCTURE ETC. 



terminal dark brown cell resting upon several elongate 

 dark brown cells_, the whole forming a very conspicuous 

 tooth standing well out from the plane of the leaf-margin. 

 Fig. 51 gives a tooth of this species from one of the late 

 Dr. Wirtgen''s specimens from the mouth of the Moselle^ 

 near Coblentz. 



In N. graminea the spines are situated on the leaf- 

 margins only (never on the midrib) at intervals equal to 

 from one half to the whole breadth of the leaf. Figs. 47-49 

 have been drawn from spines on the edge of the middle 

 portion of the leaf. Their shape is constant on the sides 

 of the lamina^ but they become longer on the sheath and 

 at the apex of the leaf. 



VII. The Leaf-sheath. 



The leaf-sheath is another important character in distin- 

 guishing the species of Naiadse^ the extent of the dilatation. 



Fig. 52. 



