Eggshells of contemporary turtles and squamate reptiles (lizards, snakes, and the tuatara) are either flexible or rigid, and usually are composed of both a fibrous shell membrane and an overlying calcareous layer. The calcareous layer of turtle eggs is calcium carbonate in the form of aragonite, whereas the crystalline material of squamate eggs generally is calcite. Both rigid and flexible shells of turtle eggs are composed of individual building blocks or shell units. Shell units of rigid shells abut tightly, and few pores penetrate the calcareous layer. In contrast, flexible shells exhibit large numbers of spaces or pores in the crystalline material. Rigid eggshells of squamates are characterized by interlocking columns of calcareous material that yield a compact, non-compliant matrix. Pores are presumed to penetrate the crystalline layer but have yet to be described in such eggshells. Flexible-shelled eggs laid by squamates exhibit the greatest variability in gross morphology of all reptilian eggs examined to date. The crystalline layer may occur as a thin, relatively unstructured crust overlying the shell membrane or may exhibit protrusions or nodes of variable morphology. In some cases, the crystalline material exists as widely dispersed aggregates separated by uncalcified areas. Pores as distinct structural entities similar to the pores of turtle eggshells do not occur, but holes and pore-like structures, as well as cracks and fissures, presumably provide for transport of gases and water between eggs and their environment.
Packard, Mary J. and Hirsch, Karl F.
"Scanning Electron Microscopy of Eggshells of Contemporary Reptiles,"
Scanning Electron Microscopy: Vol. 1986
, Article 34.
Available at: https://digitalcommons.usu.edu/electron/vol1986/iss4/34