Yesterday's post on shell repair explained that when a snail resumes shell growth following a break, new shell starts growing below the old shell and slightly behind the break. This was first shown to be case in Helix pomatia by Pollard et al.1
Diagrammatic representation of a growth break in a juvenile Helix pomatia. Top is the outside, bottom is the inside of the shell. Note how the new shell grew out from under the old shell as in model C in yesterday's post. Scale bar is 1 mm. Scanned from Pollard et al.1
Here is a real-life example that I photographed tonite. This is a fragment from a Helix lucorum shell, where there is a long break going from the top to the bottom (blue arrow). This seems to represent a growth stop rather than an injury to the shell.
Below is a highly magnified picture of the edge of the shell taken from the direction indicated by the red arrow in the picture above. The blue arrow marks the end of the break. The snail's body was along the bottom edge of the shell. The orange arrows are pointing at the long oblique overlap between the old shell (top and right) and new shell (bottom and left). Compare this picture with the diagram above.
The thickness of the shell along the photographed edge was variable; below the break it was ~0.2 mm and increased to ~0.34 mm towards the right. For the curious, I took the picture with an Olympus E-500 camera thru an Olympus SZ60 stereomicroscope.
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.