Ross: Classification of Diatoms 397 



The possibility of being misled by photographs with inadequate resolution 

 or of damaged specimens is best illustrated by particular examples. Hendey's 

 (1959) list of the species examined with the electron microscope includes an 

 indication of the ultrastructure of the valve. Both Stanroneis anceps Ehrenb. 

 and S. phoenicenteron (Nitzsch) Ehrenb. are said to have laminar valves per- 

 forated by fully open holes. His information about S. anceps is derived from a 

 picture published by Helmcke and Krieger (1953, plate 67) and that about 

 S. phoenicenteron from three pictures pubUshed by Okuno (1949, plate 3, fig. 8, 

 1952, plate 19, fig. 4, 1955, plate 9, fig. 1). In both species, however, the 

 striae consist of a series of elongated chambers with a membrane pierced by a 

 sUt on the outside and a membrane with fine pores in triangular tesselation on 

 the inside. The outer membrane is visible with a lens on negatives taken at 

 XlOOO, but not easily so, whereas the inner membrane, in which the repeat 

 distance of the pores is only about 170 A, can only be seen on negatives taken at 

 X5000. Helmcke's and Krieger's and Okuno's pictures seem to have been 

 taken at a much lower magnification than this and enlarged in reproduction. 

 Recently Helmcke et al. (1961, plates 289-290) have published pictures of S. 

 phoenicenteron showing the two membranes, but not all the detail described 

 below (p. 401). The ultrastructure can also be damaged either by chemical 

 cleaning or in fossilization. Figure 2 (p. 402) of a postpleistocene fossil speci- 

 men of S. phoenicenteron^ which may be compared with the pictures of the 

 species published by Helmcke et al., shows an example of this. 



When features are misinterpreted or imperfectly understood, and especially 

 when, in consequence, like things are considered unlike or unlike things are 

 grouped together, they will not provide satisfactory taxonomic characters. It 

 is, therefore, necessary to base any taxonomic use of the ultrastructure of 

 diatoms upon a proper understanding of that structure. Hendey (1959) has 

 presented a classification of the types of ultrastructure in which the primary 

 division is into laminar valves, consisting of one layer of siliceous substance, 

 usually perforate, and locular valves, which are formed of a double layer of 

 siliceous substance separated by vertical walls. In my opinion, however, such 

 a distinction cannot be drawn. In most cases, at least, the diatom valves are 

 pierced by chambers; these may occasionally be completely open on both sides, 

 when they may properly be described as pores, but more usually they have a 

 membrane, itself perforate, on one or both sides. In a number of cases, what 

 were originally thought to be pores have been found, when more critically 

 examined, to be closed by membranes on one or both sides. This makes it seem 

 possible that such membranes will be found to be normal throughout the 

 diatoms, and that only the mucilage pores that occur singly or in small numbers 

 in some species will prove to be true pores. What Hendey classes as partially 

 occluded perforations through a single-layered wall are exactly similar in 

 structure to what he classes as loculi open on one side; the only difference lies 

 in the closeness of their packing. His failure to realize this may be due in part 

 to the difficulty of establishing relations in depth from single electron micro- 

 graphs and his not recognizing in consequence that the membranes occluding 

 the perforations were at the level of one or other surface of the valve. Ac- 

 cordingly, the classification of Helmcke et al. (1961), based entirely upon the 

 structure of the individual chambers, is much more satisfactory. This sepa- 



