Proterovaginoceras

Proterovaginoceras (Ancient Greek for "earlier shield horn") is a medium to large sized 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.

Distribution
Fossils of the genus have been found in:
 * Loobu Formation, Estonia
 * Abastu Formation, Iran
 * Pivorjaiskaja Formation, Lithuania
 * Huk and Stein Formations, Norway
 * Gillberga, Holen and Komstad Formations, Sweden
 * China