OF THE GKNERA NAVICULA AND CYMBELLA. 419 



'Navicula Monmouthiana (Plate 27, figs. 1, 2). 



This curious fossil diatom is fusiform. Its average length is 

 xabout 07 mm. Its breadth about 001 mm. Striae very dense, 

 -parallel in the middle, then radiate up to the ends. The ends 

 not protracted. Number of striae 16 to 20 to 001 mm. As 

 ;?.ll Entoleiae Naviculae, it has rather convex valves. Fissures 

 'Comma-like turned in the same direction. Pores rather distant. 

 The axial area is regularly linear, or I should rather say parallel 

 -to the margins. 



Such is the typical form of Navicula Monmouthiana. This 

 type form is rare : the great majority of the Monmouthiana forms 

 on my Monmouth-Cherryfield slides are slightly different, already 

 showing a first variation from the type form. Although identical 

 with it in everything else, yet there is this difference, that the 

 outline or contour of the axial area is not regularly linear, but 

 .linear-lanceolate ; up to the half of the distance between the ends 

 and the central nodule this area is linear ; then it gets elongated 

 and becomes lanceolate. This difference can hardly be considered 

 unimportant or accidental, because on all such specimens it is 

 invariably constant. 



Now a lanceolate outline of the axial area is a feature peculiar 

 ^0, and very constant in, Cymhella Stodderi. On the other hand 

 the Stodderi forms are always subrostrate, whereas Monmouthiana 

 never shows any subrostration at all. I am therefore much 

 inclined to admit that the Monmouthiana forms I am speaking 

 about are not exactly N. Monmouthiana, but an intermediate 

 form between N. Monmouthiana and Cymhella Stodderi. And 

 -Irom this point of view they are the more interesting, as they 

 represent an intermediate variation : although very near to 

 the typical Monmouthiana, yet they have so to say already 

 moved towards the associated variety, the N. Monmouthiana- 

 Stodderi. 



We have thus travelled, step by step, through the whole series 

 •of these forms, which, beginning with Navicula Monmouthiana, 

 •ends with Cymhella microcephala, and which I have proposed 

 to group under the name of the " Monmouthiana Integral." 

 This whole group includes many species and varieties, yet they 

 >:are so nearly connected with one another that it is hardly 

 •.possible to say where one ends and the other begins. 



