We examine the morphometric, chemical, and physical properties of adult shells from breeding populations of Cornu aspersum maxima (Taylor, 1883), Cornu aspersum aspersum (Müller, 1774), and Helix pomatia (Linnaeus, 1758). The higher thermal requirements of the African subspecies C. aspersum maxima were confirmed by the fact that normal shell maturation, indicated by a decreasing calcium content as the snail ages, was related to an increased mean air temperature of over 22.9 °C during the breeding season. In contrast, normal shell maturation of the European subspecies C. aspersum aspersum occurred with a temperature in the range of 20.6–23.6 °C. Based on the results of texturometric analysis, shell puncture force increased with an increase in temperature during breeding. In contrast, shell puncture force decreased and collapsing force increased with increasing relative humidity. The mechanical strength of C. aspersum and H. pomatia shells was related to their chemical composition and the level of their structural maturity. Shells containing a higher percentage of calcium were characterized by lower mechanical strength than those containing a lower amount.