With respect to the terrestrial productions which lived during the Secondary and Palaeozoic periods, it is superfluous to state that our evidence is fragmentary in an extreme degree.

For instance, until recently not a land-shell was known belonging to either of these vast periods, with the exception of one species discovered by Sir C.

Lyell and Dr.

Dawson in the carboniferous strata of North America; but now land-shells have been found in the lias.