(Original Forum Question : How does a novice like me differeniate between an adult shell and a non-adult shell? Is it purely by size and if so what size makes an adult ?)
By looking and comparing as much shells as possible , in books as in reality . A mature shell usually has a good weight an full formed lip , very often young shells have nicer colours than adult ones , but I realise this information is not very practical. Some specific adult shells can look very imature . On the other hand some shells continue to grow eternally cfr. conus prometheus ( I dont know if the 10 cm plus specimens are indeed reproductive but I guess they are ) . Compare and ask advice to peers , dont by in a hurry unless from excellent origin.
(Answer by M. PVDB via the Forum)
The book you ordered is the best available for the geographic area it covers. Of course as its name indicates, it doesn't cover Atlantic cone species. Immature cone shells usally have a thinner, more fragile lip than mature shells. However, this is more helpful with some species than with others, because some cones have a rather thin, fragile lip even when mature (like C. gloriamaris). In most species though the lip is noticeably thicker and stronger in a mature specimen.
(Answer by M. Paul Monfils via the Forum)