A snail can start repairing a break in its shell one of 3 possible ways (see below): (A) New shell starts growing next to the old shell; (B) New shell starts growing above the old shell and slightly behind the break; (C) New shell starts growing below the old shell and slightly behind the break.
The red arrow points at a break in the shell; the blue arrow shows the direction of shell growth in all 3 cases. Darker brown represents old shell, lighter brown new shell. The snail's body is on the concave side of the shell.
Model B would require the mantle to flip backward around the edge of the shell. This may not be anatomically possible in pulmonate land snails. Model A would be the most economical one, because it would require the least amount of material, but to prevent the shell from falling apart, it would also require a very strong adhesion between the old and new shell edges. Note that in models B and C adhesion between the old and new shell is provided by the overlapping sections.
Pollard et al.1 showed that after a growth break in Helix pomatia, new shell grew according to model C. This is probably true for all pulmonate land snails. Below is a picture showing a long break in an Anguispira fergusoni shell. The shell on the left side of the break is "new" growth, while the shell on the right side of the break is "old" growth. Note how the new shell grew out from under the old shell as in model C above.
The aperture is on the right. The blue arrow shows the direction of shell growth.
The second post in this series is here.
1. Pollard, E., Cooke, A.S. & Welch, J.M. The use of shell features in age determination of juvenile and adult Roman snails Helix pomatia. J. Zool., 183:269-279, 1977.