She also discovered that belemnite fossils contained fossilised ink sacs like those of modern cephalopods. These include sponges, ammonites, radiolarians, brachiopods, and the belemnite "Aulacoceras". The common standard for carbon is Cretaceous Peedee formation belemnite. Passaloteuthis is a genus of belemnite, an extinct group of cephalopods. Youngibelus is a genus of belemnite, an extinct group of cephalopods. Actinocamax is a genus of belemnite, an extinct group of cephalopods. Neohibolites is a genus of belemnite, an extinct group of cephalopods. Belemnitella is a genus of belemnite from the Late Cretaceous of Europe and North America. However, scolecodont segmented worm fossils have been mistaken for belemnite hooks and vice versa. The largest belemnite known, "Megateuthis elliptica", had guards of . Megateuthis gigantea is one of the largest known belemnite species. The calcitic guard is the most common belemnite remain. The belemnite cone is composed of three parts. Successive belemnite chambers tend to increase in size exponentially. Belemnite hatchling protoconches are estimated to have been generally around .