Alectryonia fossil oyster. Alectryonia, also known as Lopha, is a genus of marine bivalve molluscs in the family Ostreidae. They are known from the upper Triassic period up to present times. The shell is characteristically curved with a strongly ribbed outer valve, the smaller inside one being flat. This specimen is from late Jurassic deposits in Tulear Province, Madagascar. Actual length is 10 cm.