摘要："Pteriomorphia, the largest subclass of the Bivalvia, includes such highly successful extant groups as the marine mussels, pearl shells, oysters, and scallops. Shells of different groups display surprisingly diverse structural types of ligaments (diagnostic traces of which are recognizable in fossils as old as the early Cambrian) and the ligaments have figured prominently in pteriomorph classifications. Within the context of general morphology, seven or eight ligament grades have been used to characterize families of pteriomorphs. But the same, or very similar, ligaments appear repeatedly as evolutionary novelties in separate taxonomic groups without evident phyletic origins or marked adaptive significance. Even their phylogenetic polarity is frequently in doubt. For example, one complex structure (duplivincular) is replaced geochronologically by a different, and simpler structure (alivincular) in families of the Arcacea, Anomiacea, Aviculopectinacea, and Pteriacea. 'Transitional' (new term), multivincular, and duplivincular ligaments all occur in the superfamily Pteriacea. Even where the geological succession of ligaments might be interpreted as indicating an ancestral-descendant relationship, the different grades of ligaments do not form gradational morphoclines. So this review is intended only to call attention to taxonomic and phylogenetic difficulties in evaluating ligament grades; we are withholding conclusions about classification until later"--P. [1].
