BILABIATE PROCESS (see Llthodesmiaceae)
FIGURE 8 Longitudinal sections of valve processes.
CLASSIFICATION (Fig. 9)
Past and current classification systems for diatoms are mainly based on phenetic data confined to the structure and shape of the siliceous parts, especially the valves (see Round et al., 1990, pp. 117-121, for a discussion on diatom systematics). However, in recent schemes other kinds of characters have been added for consideration. For instance, the type of sexual reproduction and the structure of the auxospore envelope are considered distinctive features separating the centric and pennate diatoms (Simonsen, 1979; von Stosch, 1982). Round et al. (1990) also included the type of habitat (e.g., planktonic, epiphytic, marine, freshwater) as important in ordinal descriptions.
This chapter follows Simonsen (1979, p. 45) in dividing the centric diatoms into three suborders, each characterized by the shape of the cells, the polarity, and the arrangement of the processes (process pattern). The pennate diatoms are divided into two suborders, one including diatoms without a raphe, the other one encompassing taxa with a raphe. The further differentiation of the centric diatoms into families follows Simonsen (1979) in gross features, whereas the classification of the pennate diatoms is partially from Round et al. (1990).
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