‖n. [ NL., fr. Gr. de`lta the name of the letter Δ + thy`ra door. ] (Zool.) A name formerly given to certain Silurian brachiopod shells of the genus Spirifer. [ 1913 Webster ]

Delthyris limestone (Geol.), one of the divisions of the Upper Silurian rocks in New York. [ 1913 Webster ]