Foerstellites is an Ordovician

Ordovician

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

 cephalopod genus from North America, belonging to the family Endoceratidae

Endoceratidae

The Endoceratidae is a family of large to very large straight shelled nautiloid cephalopods belonging to the order Endocerida that lived during the Middle and Late Ordovician...

, in which the siphuncle

Siphuncle

The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, and Spirula...

 takes up the entire apex.

Foestellites, named by Kobayashi, 1940, is based on the apical part of the conch, or shell, which expands rapidly as septa are added. Not a true nanno type, since the siphuncle is not truly swollen at the apex. Probably the apical end of Cameroceras or Vaginoceras.