We examined the late-stage structural evolution of droplets of a polystyrene-b-poly(methyl methacrylate) PS-b-PMMA diblock copolymer on a PS-b-PMNU "substrate". The initial droplet size distributions and shape distributions varied appreciably from one sample to another. Coarsening of the droplets occurred, wherein the average droplet size, (S), increased with time, proportional to t(beta), accompanied by a decrease in the number of droplets per unit area with time, N(t) proportional to t(-beta). The value of the power law exponent beta was found to increase as the size distribution and shape distribution of droplets decreased, from 1/10 to approximately 2/5. The droplet probability distributions, F(S/) vs S/, determined fom our data were compared with theoretical probability distributions and the results strongly indicate that the mechanism of coarsening occurs via a self-similar, dynamic coalescence process, not Ostwald ripening. The differing exponents appear to be associated with quantitative differences between the details of the coarsening process as the droplets become smaller with a narrow shape distribution. We suggest evidence of a possible dimensional crossover from 3D to 2D dynamics.