ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 797 



frustules, so that the longitudinal axis of some is only the half that 

 of others, or even less. Any one who has examined under the 

 Microscope these interesting organisms, not only on the jjrepara- 

 tions which can be bought, but better still by gathering them himself, 

 and has had the opportunity of obtaining pure gatherings, will find 

 that the cold Alpine sources will frequently have furnished him with 

 very rich and very pure collections of Odontidium hyemale Kz. Under 

 these circumstances, in examining one by one the different sizes of the 

 valve on frustules of the same species, if we take the measure of the 

 longitudinal and the transverse axes, we shall see that the dimensions 

 of the former differ considerably, in that some are only the half and 

 even a smaller fraction of the others, whilst on comparing the trans- 

 verse axes of the valves of these same frustules there is little or no 

 difference. However, no one could deny that the gradual diminution 

 of the frustules which necessarily results from their encapsulement 

 when multiplication by fission takes place, as has been completely 

 demonstrated by Dr. Pfitzer of Bonn,* ought to produce in the trans- 

 verse axes the same variation as in the longitudinal. 



Urged by the love of truth and by deep conviction, the fruit 

 of long reflection and patient research, I have been induced to explain 

 my mode of regarding the striation of the DiatomacefB, and the 

 value which should be allowed to it in the classification of species, 

 and to indicate the logical conclusions which I have thought ought to 

 be drawn from it. I know that on this point I differ from the predomi- 

 nant opinion and from the ideas of competent persons whom I other- 

 wise honour, but such is the force of my conviction that I have not 

 hesitated to brave these differences. I retain the hope that these 

 gentlemen, far from regretting my observations, will have the kind- 

 ness to take them into their serious consideration, and the more so as 

 my opinions on this subject result from the practice which I have 

 invariably employed of drawing the images of the Diatomacese by 

 means of microphotography, which, applied to all branches of 

 research, in the natural or experimental sciences, cannot fail to 

 render equally useful services. 



I think also it will be agreeable and useful to microscopists, 

 particularly to those who study the Diatomacete, to subjoin a Table 

 of the Measurement and the Number of the Striae which ornament 

 the surface of the valves in certain species. I have extracted this 

 table from my note-book, in which I put down, each time that I take a 

 measure, the number of strife corresponding to a millimetre. 



As nearly the whole of the types which I have reproduced and 

 examined have been taken from Moller's type-plate, I have adopted 

 the names and the order for the genera as they are employed in the 

 catalogue which accompanies that preparation. I do not, however, 

 wish it to be understood that I approve this order, or that I admit 

 certain modifications in the nomenclature, for instance the inclusion 

 in the genus Navicula, already so excessively extended, of all the 

 forms which Ehrenberg and Smith have relegated to the allied genus 

 Pinnularia." 



* Unters. u. Entwickel. Bacill. 

 Ser. 2.— Vol. I. 3 G 



