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The shell microstructures of 65 extant species of the family Veneridae (Bivalvia) have been described with the aid of thin sections, acetylcellulose replicas, optical microscopy, and scanning electron microscopy. Venerid shells consist of combinations of five microstructure types : composite prismatic, crossed-lamellar, fine crossed-lamellar, homogeneous and complex. The combinations of microstructure types outside the pallial line is generally uniform among individuals of a species, but some intraspecific variation in shell microstructure is recognized within inner shell layers. In general, the inner layer of venerid shells consists of homogeneous or complex microstructure. However, both microstructure types are sometimes observed among individuals of the same species, and they may even coexist within a single inner shell layer in certain species. The five microstructure types do not occur randomly within the shell, but are confined to specific shell layers. Therefore, all possible combinations of two to four microstructure types are not recognized in the Veneridae. This family shows only 12 shell microstructure combinations. Venerid shell microstructures are presently classified into three types on the basis of shell layers developed outside the pallial line. In Type I, composite prismatic and crossed-lamellar structures coexist. In Type II, crossed-lamellar structure is not developed in any shell layer. In Type III, composite prismatic structure is not recognized in any shell layer. These three shell microstructure types are divided into nine subtypes on the basis of the arrangement of structural elements (e.g., first order crossed-lamellae or prisms) and the mode of ontogenetic appearance of each microstructure. Most venerid subfamilies are each represented by a single shell microstructure type. However, two or three shell structure types coexist within the subfamilies Chioninae, Tapetinae, and Callistinae. The relationship between shell structure and shell morphology was also examined in this study. The ratio of shell width to maximum height and the position of maximum width of the shell is relatively uniform among the examined venerid species. On the other hand, the lateral profiles of venerid shells are variable and can be related to shell structure types. Species with shell structure Type I are generally represented by circular to ovate shells. Species with shell structure Type II have elongate to elliptical shells. Species with shell structure Type III are generally trigonal in shape. Moreover, shells representing the three shell structure types have distinct shell surface features. The surface ornamentation of most Type I species consists mainly of raidal ribs, concentric ridges, or cancellate sculpture. In shell structure Type II, concentric ridges comprise the dominant surface ornamentation. The surfaces of type III shells are predominantly smooth. As to geographic distribution, venerids with composite prismatic structure (Types I and II) show a more northerly distribution around the Japanese Islands than venerids lacking composite prismatic structure (Type III). Judging from available paleontologic data, Type III venerid species (lacking composite prisms) appeared earlier than Type I and Type II species (with composite prisms). Moreover, among type I species, crossed-lamellar structure tends to be replaced by homogeneous structure as time advances. Inferred phylogenetic trends in venerid shell microstructure are as follows : The subfamilies Cyclininae, Dosiniinae, Venerinae, Callistinae, and Pitarinae appear to be microstructurally primitive taxa in the family Veneridae. Saxidomus and Callista (Ezocallista) are considered as the incidental group to the Venerinae. From ancestors in the group represented by the Venerinae, Saxidomus, and Callista (Ezocallista), the Chioninae evolved through replacement of crossed-lamellar structure with homogeneous structure in the middle or outer shell layer. The subfamily Tapetinae and its Type II structure then evolved with the disappearance of crossed-lamellar structure. The subfamilies Meretricinae and Sunettinae evolved from the Callista (Callista) group, and the Lioconchinae evolved from the Pitarinae. The ancestral group of the subfamily Circinae and the subgenus Gomphina (Gomphina) is uncertain, but they may have evolved from the Pitarinae, judging from their geographic distributions. |
