FIGURE 4. Strict consensus tree of the 3 most parsimonious trees resulting from cladistic analysis herein, showing inferred relationships among stenothecids, bivalves and rostroconchs. Data matrix in Table 3; characters and character states listed in Table 4. Letters indicate character transitions. Key: a, pegma or pegma-like structure, curved ventral margin, lateral compression, laminar inner shell layer; b, reduced shell coiling; c, loss of spiny (lamello-fibrillar) shell microstructure; d, loss of pegma; e, raised sub-apical lip; f, straight dorsal margin, sub-apical width greatest; g, calcitic semi-nacre (or similar) inner shell layer; h, prominent shell pores, aperture constriction near sub-apical margin.