(Text-figs. 1-4) FOSSIL OSTRACODSDuring a recent study of the ostracod faunas of the Portland and Purbeck Beds of the Aylesbury district (a full account of which is in preparation) a euryhaline ostracod fauna was reported. The ostracods in question are Pabanella polita Martin and Pabanella ansata (Jones). A study was made of their vertical distribution at three localities: (a) at the Bugle Pit, Hartwell, It was found possible to subdivide the beds at these localities into a number of facies characterized in the main by ostracod faunas. At the base are (i) the massive cream coloured limestones of the Portlandian with marine ostracods and referred to as the Portland facies. Above these are (ii) laminated marls and limestones with a mixture of marine and euryhaline ostracods referred to as the marine Purbeck facies, (iii) laminated marls and limestones with euryhaline ostracods referred to as the brackish Purbeck facies, and (iv), marls and limestones becoming sandy upwards with freshwater-oligohaline ostracods referred to as the freshwater Purbeck facies. These facies demonstrate a gradual change from marine Portland conditions to the more or less freshwater beds of the Purbeck.Pabanella was found in facies (i), (ii), and (iii), and it was noticed that there seemed to be a slight difference in size between specimens in each of the three facies.All the specimens of Pabanella were measured from the bed in each facies with the largest number of specimens; the results for each locality are shown in Figs. 1 and 2. Histograms are drawn for the lengths of the ostracods from the various beds at each locality. Inspection shows that the range oflength of the instars overlap. To calculate the separate statistics for the instars represented, the results were plotted on arithmetic probability paper. A normal distribution plotted on this paper produces a straight line, and if a bimodal or polymodal distribution is made up from a series of normal distributions then a curve will be produced which is the resultant of two or more straight lines. Using a method described by Harding (1949) the position of these

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