The curious phenomenon of shell torsion arose independently among members of two separate lineages within the Pteriomorpha (Bivalvia), the Arcidae and the Bakevelliidae. In torted bivalves the shell is twisted about the hinge axis normal to the antero-posterior direction, and the usually two-dimensional plane containing the commissure is deformed into a complex three-dimensional surface. The fabrication of shell torsion is particularly amenable to mathematical analysis, and by utilizing computer simulation it is possible to tort a normal bivalve by introducing ‘errors’ of a particular nature into the morphogenetic program. Analysis of the pattern of torsion within the Arcidae and Bakevelliidae, and the examination of modem untorted arcids, indicate that the interference of the byssus with the normal bivalve morphogenetic program played a definitive role in the origination of the torsion. Though quite different adaptive pathways were utilized by the two groups, the fabricational principle producing the torsion remained the same. The resultant torted morphologies observed in the Arcidae and Bakevelliidae can be understood in terms of the interaction between the adaptive significance of the torsion and the historical, or preexisting morphology of the animals involved.