Delthyris

from The Collaborative International Dictionary of English v.0.48

Delthyris \Del*thy"ris\, n. [NL., fr. Gr. de`lta the name of the
   letter [Delta] + 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]