Some families consist of more genera than those dealt with in this chapter. In these cases, only characters common to the genera presented are listed. Keys to genera and/or a list of generic characters (for monotypic genera the description of the generitype) characterize each genus. Keys to species and/or a list of characters distinguishing the species of a certain genus describe each species. For the larger genera the species are grouped (A, B, C, etc.) according to a few common characters.

When up to date information from the literature is lacking, morphometric and distributional data from Hustedt (1930,1959,1961), Cupp (1943), Hen-

dey (1964), and the authors personal observations are given. Few planktonic species and localities have been regularly investigated over a longer period of time. It is therefore seldom worthwhile to go into detail concerning species distribution. We accordingly confine ourselves to the simplest system, viz. three main biogeographical provinces as distinguished for marine plankton communities by Zeitzschel (1982). These are the circumglobal warm water region, a northern cold water region, and a southern cold water region. This classification is fairly consistent with the results of a detailed investigation of 26 marine planktonic diatom species (Hasle, 1976a). The diatoms classified as "cold water" species are often associated with sea ice. Those classified as "warm water" species may have a much wider latitudinal range than that of the warm water region due to transport by currents out of their reproduction areas.

The catalogue by VanLandingham (1967-1979), although not always followed, was consulted for references to publications and synonyms. The generi-types are from Farr et al. (1979, 1986), Greuter et al. (1993), and from more recent literature with some modifications (P. C. Silva, personal communication). The types are cited as they were originally named. In cases of synonomy, the names now regarded as the correct names (the names recommended to be used) are also listed.

Taxa, names, and combinations previously unpublished and used under Description of Taxa are formally described in the Taxonomic Appendix.

The synonomy list is meant as an aid to avoid confusion concerning names commonly used in the literature. Principally, one or more common synonyms, not necessarily including the basionym, are presented. In general, synonymy information too recent to be found in Hustedt (1930, 1959, 1961), Cupp (1943), and Hendey (1964) has been updated. For further information the reader is referred to these publications as well as to synonyms and literature references in the present chapter.

As for the plates presented in this chapter, the sources are given for figures redrawn from the literature; if no source is indicated, the figure is original. In the Thalassiosiraceae, strutted processes are illustrated by a dot, sometimes with a short line attached to indicate the longer extension of the process, and labiate processes are illustrated by a line. In other diatoms, the labiate processes are illustrated by dots, triangles, etc. depending on their relative size and shape. In a series of figures with the same magnification, the scale bar is marked only for the first figure.

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