418 SIR NICHOLAS YERMOLOFF OX SOME INTERMEDIATE FORMS 



jiieus, very clearly punctate. Its chief distinctive feature is the. 

 area, which is linear-lanceolate, more or less linear and more or 

 less lanceolate. The Rev. Francis WoUe, in his Diatomaceae of 

 North America, gives a figure of it on Plate LXIII, fig. 34, but, 

 this figure is not good : it is too broad, too naviculoid, and the 

 ends are too rostrate. The area is shown too broad. However,. 

 there seems to be no other figure extant. Cleve mentions a very 

 true and very important fact : that Cymhella Stodderi is nearly 

 akin to Navicula Monmouthiana, and so it undoubtedly is. In. 

 fact, as soon as Cymhella Stodderi loses the subrostration of its., 

 ends and the lanceolate outline of its area, it merges immediately 

 into Navicula Monmouthiana. 



Navicula Monmouthiana- Stodderi (Plate 27, figs. 3, 4). 



This is a form intermediate between Cymhella Stodderi and' 

 Navicula Monmouthiana. The evolutionary transition from is. 

 Kavicula to a Cymbella is already well marked : the valves have 

 acquired asymmetry and a slight tendency to subrostration. The- 

 striae are more punctate and the axial area is broader and more- 

 lanceolate than on Cymhella Stodderi. 



I now come to the final member of the whole series I have- 

 examined, to Navicula Monmouthiana. 



As I have already stated, this last term of the series seems tt^ 

 me to be the prototype of them all, I mean the type to which 

 all the others are, so to say, pointing. In my opinion, it is the 

 ancestral Pliocene form. We ought, therefore, not to climb up- 

 to it from Cymhella microccphala, but on the contrary descend 

 the ladder in the opposite direction, admitting Motimouthiana as 

 the first, and microccphala as the last, member of the series.. 

 In my opinion, Navicula Monmouthiana seems to have been the 

 fossil ancestral parent, from which the whole tribe has derived 

 if not to say degenerated. Now it is well known that several, 

 authors admit that (-ymbellae are degenerate Naviculae. This 

 admission seems plau:ible. I do not like to use the term 

 " degenerate," because we cannot know the trend and direction 

 of evolution, yet it appears quite natural to admit that Navicula 

 Monmouthiana is, so to say, a perfect form, whereas the Cymbellae- 

 are imperfect forms. 



