Internally, the shell and mantle have;
1: a brown banded aperture rim made of the exterior shell-layer that is secreted by the mantle-edge that has mustard brown pigment dorsally, but is translucent white showing dark brown shell when viewed ventrally, as in this image.
2: a wide, matt white, peripheral ‘skirt layer’ of shell, showing faintly the exterior pattern; secreted by the mantle-skirt that has emerald green pigment dorsally which looks blue-green ventrally as in this image.
3: a very thin whitish mantle-attachment scar, most distinct at the anterior where it is not flanked by the scar of the pedal retractor muscle.
4: the narrow, glossy, white U-shape, pedal retractor muscle scar made by the muscle attachment.
5: an amphora shaped, chocolate brown area enclosed by scars 3 & 4; secreted by the green mantle over the visceral hump.
6: a cream/orange/pale brown patch at the vertex, secreted by the less intensely green mantle in that area.
When viewed ventrally on a live specimen, only the mantle skirt is visible and the colours are affected by the adjacent shell interior and by being seen through the mantle
For wider views see images Tt15 (mantle) 15 Testudinalia testudinalis. Shell length 14.1mm. Littoral. Moray Coast, Banffshire, Scotland. May 2017. Leg. S. Taylor. and Tt16 (shell) 16 Testudinalia testudinalis. Length 14.1mm. Littoral. Moray Coast, Banffshire, Scotland. May 2017. Leg. S. Taylor. .
APPENDIX re range advance/retreat: flic.kr/p/2jW74ig