Morphological evidence indicates that within the snake family Boidae the subfamily Pythoninae is ancestral to the Boinae. The python-boa transition in evolution involves a peculiar rotation of the premaxillary bone. During rotation the ventral process of the premaxilla, the processus palatini, is largely obliterated but reconstituted in later Boinae. There exist indications that the pattern of the processus palatini is less controlled or stable in boine snakes than in pythonines. Pattern instability can be the result of several factors which include relaxed selection for precise form and a disruption in the developmental foundation of the feature which has not fully been reversed. These possible factors, though fundamentally different, need not exclude each other. It is surmised that the pattern instability in boines is related, at least in part, to the disruption of a morphogenetic paradigm affecting premaxillary shape.