Isognomon alatus is a member of the family Isognomonidae. Its' shell is fan-shaped appearing extremely compressed and thin-walled. The hinge is straight and elongated (Siung 1980). The color of the flat tree oyster is usually drab greenish white to purple-black with flaky co-marginal lamellation (i.e. thin layers or plates). The interior of the shell (nacre) is yellowish with a purple-brown stain. The border is darker (Mikkelsen and Bieler 2008). The soft body of the animal is oval and covered by an opaque mantle (Siung 1980).