This paper discusses postembryonic early mode of life in Carboniferous
 goniatites based on analysis of ammonoid assemblages and calculation of total
 shell densities of actual  specimens. Co-occurrence of embryonic and
 postembryonic shells appears common in Carboniferous Goniatitina, as confirmed
 in Homoceras, Homoceratoides, Vallites, and Reticuloceras from the Middle
 Carboniferous (Namurian) of west Ireland and England, in Tumulites,Cravenoceras
 and Eumorphoceras from the Upper Mississippian (Chesterian) of Texas, and in
 Aristoceras and Vidrioceras from the Upper Pennsylvanian (Virgilian) of Kansas.
 Embryonic and postembryonic shells of these genera occur in ammonoid-packed
 calcium carbonate concretions (bullions) or in black bituminous limestone beds,
 together  with rare shells of bactritoid cephalopods, brachiopods, bivalves and
 gastropods.  They appear to not have been transported a lang distance be-
 cause of the absence of a preferred orientation, size sorting and apparent
 shell abrasion or fragrnentation. Buoyancy calculation of specimens of 12
 species belonging to 8 genera including Homoceras, Aristoceras, and
 Vidrioceras from several fossil localities shows that the total shell densities
 at hatching remain almost constant among these species and can be
 approximated to that of seawater (=neutral buoyancy).  These lines of
 evidence strongly suggest a similarity in the biostratinomic properties
 of empty shells between post-hatching juvenile stage and middle-late
 ontogeny.  In all probability, in most Carboniferous Goniatitina, a newly
 hatched arnmonoid had a nektobenthic or nektic mode of life, as has
 been postulated for adult animals.