When M. thyroidus becomes an adult, it stops growing & forms a reflected
lip. Prior to that, as Thomas Watters explained, periostracum forms first &
extends over the already formed shell. On the other hand, Anguispira never
really stops growing, especially if you keep them in captivity. So, there is
always a very narrow strip of periostracum extending over the lip. In
juvenile shells, regardless of species, the shell around the lip is always
soft & fragile.

Aydin

> -----Original Message-----
> From: James M Cheshire [mailto:[log in to unmask]]
> Sent: Saturday, April 29, 2000 23:52
> To: [log in to unmask]
> Subject: perio
>
>
> Dear all,
> I have something to report. I have been researching some land
> snails from
>
> around the central Ohio area, and I have made an interesting
> discovery;
> the shells of a couple species (Mesodon thyroidus, Anguispira
> alternata)
> with the living animal have an "extension" of periostracum that goes
> beyond
> the lip, apparently protecting the actual shell edge from damage. In
> Mesodon
> thyroidus, this "perio sheath" is present only in juveniles; the adult
> shell has a thick,
> ringlike lip. Anguispira alternata, on the other hand, has
> this feature
> all the time. Now, I
> have two questions; first, has anyone else on the list noticed this on
> snails? Second, if this
> perio is damaged, will it create a growth mark, or must the
> shell itself
> be damaged?
>
> Best Regards,
> Jimi
>
> Ohio Molluscan Studies
> c/o James M. Cheshire
> 427 E. College St.
> Granville, Ohio 43023
> USA
>