The costellae of a population of brachiopods can be quantitatively described by plotting the total number of costellae on a valve against the length of its margin. The resultant curve describes the continuous change of the average distance between adjacent costellae during growth. When costellae first appear, this distance has the value of about 0.06 mm and it increases during growth to various degrees in different brachiopod species. If it is assumed that costellae mark the sites of setae, then the magnitude of the distance between adjacent costellae can be discussed in terms of the function of setae. The magnitude of the parameters of the distribution of setae and punctae are compared to the silt/sand dividing line.

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