SUMMARYOn the basis of the characterization of 126 strains representing 46 different species of Pseudomonas and 6 species of Aeromonas obtained from different culture collections, a proposal for the taxonomy of the genus Pseudomonm is made. By using various old and new diagnostic methods some 90 different morphological and cultural characters, many obtained by modification of methods and media, were investigated. The iodoacetate test was developed to distinguish between oxidative and fermentative utilization of glucose.Results were evaluated by the use of punched cards and by the method of strips. The relationships between the established species of Pseudomoltas and Aeromonas were shown by using an Adansonian principle and by taxonomic models. On the basis of the results obtained proposals for the definition of the genus Pseudomonas and for re-descriptions of Pseudomonas species are given and the relations to related genera are shown. In the definition of the genus Pseudomonm an attempt to find a more general principle for subdivision within the genus was based on the type of glucose utilization and on the relationships of the species expressed in some other features. This proposal about the taxonomy of pseudomonads is compared with that of Rhodes (1959).

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SUMMARY:The making of models for taxonomic purposes and their use is described, and the fidelity with which they can represent the taxonomic relations between bacteria is discussed. These models are particularly useful for teaching purposes.The use of equally weighted features in creating taxa allows the comparison of organisms by simple mathematical methods in which the similarity between two strains can be expressed as a percentage; the strains can then be grouped into taxa whose members show high similarity with each other. This method was described by Sneath (1957a, b), who used it for a study of the genus Chromobacteriurn. It also permits solid models to be made of taxonomic groups, in which the spatial arrangements of the component parts of a model is an expression of the relation of organisms or groups of organisms within the taxonomic group. The preparation of such models is the subject of the present paper. METHODSThe meaning of the symbols used is given in Sneath (19573). Two groups of bacteria were used for the models illustrated here. The first was the family Enterobacteriaceae as it is a t present generally defined. The second was the group of chromobacteria studied by Sneath (1957 b).Enterobacteriaceae. In Table 1 are given the features currently used for the differentiation of a number of groups of the Enterobacteriaceae under names which are commonly given them. From Table 1 values of S were calculated by the method of Sneath (19573); these values are shown in Table 2. Since the data in Table 1 do not represent the features of single strains, a slightly different convention for obtaining S was used; +, ( +) and x were counted as positive, and d was counted as NC (no comparison). The data in Table 1 It should be noted that the tests used are selected, and therefore are biased; the use of these tests cannot show whether the groups are valid taxa, since this prejudges the question. They are used only as an illustration of the use of models, and the model is not a model of the ideal taxonomy of the family, but only of the current ideas about the taxonomy of the family. The model shown (PI. 1, figs 1, 2) was made with distance d proportional to 1 -8 (see below).(1 9 57).