K. C.  Nixon and Davis, J. I., ?Polymorphic taxa, missing values and cladistic analysis?, Cladistics, vol. 7, pp. 233-241, 1992.

K. C.  Nixon and Davis, J. I., ?Polymorphic taxa, missing values and cladistic analysis?, Cladistics, vol. 7, pp. 233-241, 1993.