The present paper is the first report of a series of microscopic observations of the hings part of various arcacean species. By the optical microscopic observations, the type of structure of the tooth in the genus Anadara differs both from the "complex structure" composing the inner layer of the shell and from the "crossed lamellar structure" of the outer layer. For this structure of the tooth, the "elliptic lamellar structure" is newly proposed. The hinge plate underlying the tooth is composed of the "crossed lamellar structure". The elliptic lamellar structure is constructed of the aggregation of the basic units of very small lamellae. Although these lamellae are the same in size and form as the "second order lamellae" which are the basic units of the outer shell layer, the difference between the shell layers and the tooth, can be recognized in the mode of aggregation of the second order lamellae. The tooth structure is diagramatically shown in the stereographic model (Text-fig.3) depicted on the basis of the observation in the three main sections. There are described another structure of the striations on the side wall of tooth and the initiation of the growth of the tooth on the lateral margin of hinge.

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