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Fifty characters of extant advanced snakes were selected for phyletic analysis, with the primary aim of determining evolutionary paths of the venomous taxa. The characters chosen showed roughly equivalent variation. States of characters were distinguished with reference to range of variation at species and generic levels. The heterogeneous, widespread and abundant family Colubridae was designated as representing the ancestral group. Direction of change in states was then determined by reference to colubrid conditions. Criteria used for judgement of direction were (1) uniqueness, (2) relative abundance, (3) correlation of derived states, (4) morphological specialization, (5) ecological specialization, (6) geographic restriction, (7) closely related taxa, and (8) correlation of applicable criteria. Two other criteria, (9) genetic structure and (10) fossil record, were not applicable.

Characters used as examples of the approach are number of palatine teeth and the pattern of the head shields. In the latter, the derivative states are correlated with particular modes of life (aquatic, fossorial and terrestrial). The familial level taxa show rather different frequency distributions in respect to the four states of this character, with one derived state unique to the viperids.

The second character, number of palatine teeth, was divided into classes by using a span covering most intraspecific variation. The resulting classes were lumped into four states emphasizing the classes of low numbers of teeth. The extreme derived states are correlated in the colubrids with distinctive modes of life (burrowing and digging) and with functional morphological specializations. The distribution of the states shows a trend toward low numbers of teeth in all the venomous families.