The rate of lobe division during thallus enlargement in Xanthoria elegans and Diploicia canescens is considered in geometric terms. The frequency of lobe engulfment by neighbouring lobes was measured, was fitted graphically to a simple equation and was found to result from constant lobe division with thallus size. Values for a constant in the equation and derived from the observed frequency of lobe engulfment provided a measurement of a new morphological parameter: ratio lobe length to width. Greatest departure from circularity in small thalli of D. canescens occurred when thallus size did not permit lobe competition. Increasing lobe engulfment and thallus circularity in larger thalli appeared to be correlated. Circularity in crustose thalli without lobes is discussed with definitions of ‘primary’ and ‘secondary’ areoles.