Agerostrea
Class BIVALVIA Linné, 1758
Subclass PTERIOMORPHA Beurlen, 1944
Superfamily OSTREOIDEA Rafinesque, 1815
Family OSTREIDAE Rafinesque, 1815
Subfamily LIOSTREINAE Vyalov, 1983
Genus Agerostrea Vyalov, 1936

The species is distinguished from falcate congeners and species of Rastellum (Faujas-Saint-Fond, 1799) by its smooth central field in both valves and general absence of shell chambers (with respect to Rastellum). According to Stenzel (1971), chomata are tubercle-like and surround the commissure; however, this is a character Malchus et al. (1994) found on Rastellum diluvianum (Linné) rather than Agerostrea.
Agerostrea is a Late Cretaceous (and possibly Pliocene) genus while Rastellum ranges from the Callovian (end of the Middle Jurassic) to the end of the Creatceous, so they overlap.
Although they are both in the true oyster family Ostreidae, they belong to different subfamilies

The clue someone used is: Agerostrea has shorter ribs, and even smooth surfaces (externally and internally). Rastellum have long ribs, forming a sort of spike.
As example, In fig2 Agerostrea is on the left side.


56mm sur gangue
Campanien moyen / Barzan / Charente-Maritime / France

Late Cretaceous-Maastrichtian
Maastricht Formation



参考:The first record of Agerostrea ungulata (von
Schlotheim, 1813) (Bivalvia: Ostreoidea)
from the Upper Maastrichtian of KwaZulu,
South Africa, with a discussion of its distribution in
southeast Africa and Madagascar
龘衋