Proterovaginoceras (Ancient Greek for "earlier shield horn") is a medium to large sized[ambiguous] endocerid (endocone-bearing orthoconic nautiliod) from the Early and Middle Ordovician included in the family Endoceratidae.
Proterovaginoceras has a straight shell with a circular cross section, straight sutures, and a ventral to central, nanno-type, siphuncle which fills the entire apical part of the shell. Septal necks are macrochoanitic, up to two camerae (chambers) long; connecting rings, one chamber in length line the inside of the necks. Endocones are long and slender, fill the entire bulbous, nanno-end, of the siphuncle.
Proterovaginoceras was named by Reudemann in 1905. Dideroceras Flower 1950 and Chisloceras Gortani 1934 are probably synonymous equivalents.