This is the second paper of a series of studies on the tooth structure in various Arcacean bivalves, reporting microscopic observations of thin sections of the teeth area in the genus Arca. The tooth structure of genus Arca is different from those of many other shell structural patterns which have ever been studied by some investigators. In the genus Arca, the tooth is consisted of the aggregation of the second order lamellae, whose characteristic pattern is well demonstrated in a tangential thin section showing the simple and parallel aggregation of the fibrous microcrystals. The crossed lamellar structure is the major constituent of the outer shell layer in Arca and is also composed of the aggregation of the second order lamellae. However, from another point of view, the tooth structure and the crossed lamellae are clearly different from each other, especially in the aggregation pattern. That is, one of the latter is composed of the crossed aggregation of the second order lamellae, while the former is the simple aggregation of plate like lamellae of the second order lamellae. Therefore, the name of "simple lamellar structure" is newly proposed for the tooth structure of the genus Arca, whose idealised streographic model is shown in text-fig. 2. Another part of the tooth area, such as the hinge plate and the hinge socket are composed of crossed lamellar stucture and the irrgular crossed lamellar structure, respectively. These differences of the structures in the tooth area are due to the aggregation pattern of the second order lamellae. Therefore, the second order lamellae is considered to be the fundamental unit of these structures. From the preceding fact, it can be pointed out that the tooth structure of genus Arca having the simple lamellar structure, apparently differs from that of genus Anadara (MANO and OMORI, 1969), which has an elliptic lameller structure. Especially, the tangential section of the tooth area shows such difference as the former consisting of the simple aggregation of fibrous microcrystals in contrast to the latter having elliptic aggregation of the same. Morever, hinge plate of genus Arca is less faintly developed than that of Anadara. The type of the origination of the hinge tooth at the early stage of the tooth development, and the structure of the striation of the sides of the tooth are also reported.