Once you're in the field, though, your eye learns to distinguish the rounded top of a fossilized urchin or the curving edge of a nautiloid.
32.
It was previously believed that these represented the most primitive form of nautiloid, but it is now known that the earliest nautiloids had shells that were slightly curved.
33.
""'Megaloceras " "'is a genus of the nautiloid order, Oncocerida that lived during the Silurian period of the Paleozoic.
34.
""'Procymatoceras " "'is a nautiloid cephalopod from the Middle Jurassic with a large, tightly involute, rapidly expanding shell.
35.
"' Eothinoceratidae "'is a family of Lower Ordovician nautiloid cephalopods included in the Ellesmerocerida originally established for the genus " Eothinoceras ".
36.
""'Heminautilus " "'is an extinct nautiloid genus from the nautilacean family Cymatoceratidae that lived during the early Cretaceous ( 1964 ).
37.
""'Grimsbyoceras " "'is a nautiloid genus included in the Oncocerida order of the family Acleistoceratidae that lived during the Middle Devonian.
38.
""'Karoceras " "'is an oncocerid nautiloid that lived during the Silurian and possibly Early Devonian, type genus for the Karoceratidae.
39.
This adds new chambers to the shell, which can be clearly seen in cross-sections of the shell of the living nautilus, or in ammonoid and nautiloid fossils.
40.
""'Paldoceras " "'is a nautiloid cephalopod found in the middle Ordovician of Estonia and Sweden, included in the discosorid family Apocrinoceratidae.
