TERMINOLOGY AND CLASSIFICATION OF DIATOMS. 



93 



Fig. 25. 



-Section of Navicula (after 

 W. Prinz). 



Fig. 25 (for which we are indebted to Mr. W. Prinz) is a faithful repre- 

 sentation of this section ; a is the 

 raphe ; b, c, d one of the costre of 

 the valve ; e the bevelled margin of 

 the valve ; ff the central nodules ; 

 g g the connecting parts. The 

 magnification is 1500 diameters. 

 The portions of the plate that are 

 printed black are of a brown colour 

 in transmitted light. 



The raphe, which is also called 

 the median line, is generally inter- 

 rupted by a central nodule and two 

 terminal nodules. 



The true raphe, such as exists in the Navicula, must be distin- 

 guished from the pseudo-raphe, which is a simple line or blank space 

 and is without a central nodule, such for example as is seen in KapJioneis. 

 It is probable that all diatoms have more or less perfect raphes. 

 In the sub-family I. Raphidiese, the raphe is quite evident, central or 

 sub-central, but it sometimes only exists on one of the valves of the 

 frustule, as is the case with the Achnanthes (pi. 8, f. 323-337) and the 

 Cocconeis (pi. 8, f. 338-345). 



In the sub-families II. Pseudo-Raphidieae and III. Crypto- 

 Raphidiese it is obscure, marginal or sub-marginal ; moreover the greater 

 number of the valves of the sub-family II. have a central or sub-central 

 pseudo-raphe. 



The raphe may assume one of the following forms: 

 1st. Simple as in most of the Navicula (plates 2-5). 

 2nd. Bijurcated, displaying in addition to the principal raphe a second 

 raphe much narrower, and starting from the principal raphe, to which it also 

 returns. Ex. Slauroneis acuta (pi. 1, fig. 51), Navicula nobilis (pi. 2, fig. 68). 

 3rd. Double placed on each side of the central and terminal nodules 

 as in the Vanheurckia (pi. 5, f. 249-251). 



The nodules are generally thickenings of the valve, which probably 

 are the means of strengthening them. 



In the Raphidiese there are usually three nodules : the one placed in 

 the middle of the valve is called the central ?iodule, and the other two 

 situate near the extremities are called terminal nodules. 



The central nodule is defective (or rudimentary) in the Amphiplcura 

 (pi. 5, f. 253), and it is duplicated in the Berkeleya (pi. 5, f. 254). 



