Abstract Tokyo is situated in the southern part of the Kanto Plain, the largest plain in Japan. It is filled with thick marine Quaternary sediments deposited in the Palaeo-Tokyo Bay. The concept of a Palaeo-Tokyo Bay was proposed by Hisakatsu Yabe in 1913 and 1914 , based on molluscan fossils, geography and tectonics. Palaeo-Tokyo Bay used to open to the east and, at the time of the high sea-level phase, perhaps also to the south, whereas the modern Tokyo Bay opens to the south. The Paleao-Tokyo Bay concept is supported by recent sedimentological evidence, sequence stratigraphy and tectonic studies, and is considered to have formed in a forearc basin near the triple junction of plates (the Pacific, Asian and Philippine plates) and trenches (Japan Trench, Sagami Trough and Izu-Ogasawara Trench) in the Quaternary. The bay is unique and shows evidence of repeated differential vertical movements during its formation.

Abstract Pteriomorphian spermatozoa, like those of most other bivalves, are of the classic aquasperm type (conical acrosomal vesicle, short to rod-shaped nucleus, short midpiece composed of two centrioles and a ring of spherical mitochondria, a simple flagellum). Whereas most other bivalve subclasses show at least some defining acrosomal feature(s), this does not appear to be the case within the Pteriomorphia. While this could indicate non-monophyletic status, it also correlates with the fact that the Pteriomorphia are a very old and very successful group of bivalves. Acrosomal similarities suggest a close link between the Ostreoidea and Limoidea (acrosomal vesicle with wedge-shaped apical zone; radiating plates present but not well developed); and between the Pterioidea, Pinnoidea and Pectinoidea (dense anterior layer; very well developed radiating plates). For supposedly closely related taxa, the Arcoidea and Limopsoidea (both Arcoida) differ markedly from each other in acrosomal shape and substructure. The affinities of the Anomioidea and even more so the Mytiloida remain uncertain, the latter possibly connected with the Pterioida or, more likely, removed from the rest of the Pteriomorphia (mytiloid acrosomes show concentric lamellae). A very close relationship between the Pectinidae and Spondylidae of the Pectinoidea is demonstrated (dense anterior layer of acrosome recurved). Within the Mytilidae (Mytiloidea) there is substantial variation in sperm morphology between supraspecific taxa especially at the subfamial level.