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Plates I-II, Fig. 1) Scanning electron and optical microscopic examination of the hinge apparatus and primary ligament pit of two Recent mytilids (Mytilus edulis and Modiolus modiolus) revealed a similar sequence of ontogenetic changes in both species from the prodissoconch I stage through metamorphosis. Hinge-line dentition was absent at the prodissoconch I stage. Provinculum length and complexity increased throughout larval development with progressive lateral thickening characteristic of the family Mytilidae. The number of teeth in prodissoconch II and early dissoconch stages increased significantly with both total shell and provinculum length. Ridged structures on anterior and posterior surfaces of hinge teeth became increasingly apparent with shell growth. In neither species did lateral teeth develop in larval or early post-larval individuals. Significant differences at the population level were found between Mytilus and Modiolus after regression and quantitative comparison of each of the following: larval length and provinculum length; larval height and provinculum length; provinculum length and number of teeth; and larval length and number of teeth. Such differences can be used for unambiguous specific identification of single specimens of these two mytilids. Ligament pits were first observed in Mytilus and Modiolus at shell lengths of 260 and 285 /Jtn, respectively. These observations, when coupled with detailed examination of published micrographs of other bivalve species, provide evidence that morphological structures heretofore referred to as 'larval ligament pits' are post-larval features.It is suggested that routine optical microscopic examination of the hinge-line morphology of prodissoconchs and early dissoconchs may facilitate specific identification of many Recent and fossil bivalves. Inasmuch as the bivalve ligament pit does not appear to develop prior to metamorphosis, presence or absence of this structure may be of assistance in ecological and palaeoecological studies for differentiating larval and postlarval individuals.

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