A theoretical morphologic model defining ligament formation in the Bivalvia is introduced. It is based on the spacing of a lamellar layer, the spacing of a fibrous layer, and the relative growth rate of the expanding ligament with respect to enlargement of the ligamental area. Most of the diverse patterns of bivalve ligaments are successfully modeled by computer simulations. Wide intraspecific variation of the ligamental pattern is observed in an arcid species, Tegillarca granosa. This appears to be a consequence of allometric change of morphogenetic parameters during growth, adjusted to maintain the relationship between ligament strength and shell weight. The distribution of actual ligaments, which does not fill the theoretical morphospace, shows potential evolutionary pathways of bivalve ligaments. Thus, it implies phylogenetic relationships between ligament types from the viewpoint of pattern formation.