genus of extinct gastropods (snails) that occurs as fossils in rocks of Silurian to Permian age (444 million to 251 million years ago). Its distinctive shape is easily recognized. The caplike shell is high and broad anteriorly. The posterior portion of the shell, at the apex, is slightly coiled in an asymmetrical fashion. Frequently, the front portions of the shells are broken, though the posterior sections are relatively well preserved. Platyceras is particularly abundant in Devonian deposits (359 million to 416 million years old).