The remains of land and fresh-water shells are not much more frequent than those of insects. Like them, too, their forms are very stable, continuing unchanged through several geological {169}periods. In the Pliocene and Miocene formations, most of the shells are very similar to living species, and some are quite identical. In the Eocene we meet with ordinary forms of the genera _Helix_, _Clausilia_, _Pupa_, _Bulimus_, _Glandina_, _Cyclostoma_, _Megalostoma_, _Planorbis_, _Paludina_ and _Limnæa_, some resembling European species, others more like tropical forms. A British Eocene species of _Helix_ is still living in Texas; and in the South of France are found species of the Brazilian sub-genera _Megaspira_ and _Anastoma_. In the secondary formation no true land shells have been found, but fresh water shells are tolerably abundant, and almost all are still of living forms. In the Wealden (Lower Cretaceous) and Purbeck (Upper Oolite) are found _Unio_, _Melania_, _Paludina_, _Planorbis_, and _Limnæa_; while the last named genus occurs even in the Lias. The notion that land shells were really not in existence during the secondary period is, however, proved to be erroneous by the startling discovery, in the Palæozoic coal measures of Nova Scotia, of two species of Helicidæ, both of living genera--_Pupa vetusta_, and _Zonites priscus_. They have been found in the hollow trunk of a _Sigillaria_, and in great quantities in a bed full of Stigmarian rootlets. The most minute examination detects no important differences of form or of microscopic structure, between these shells and living species of the same genera! These mollusca were the contemporaries of Labyrinthodonts and strange Ganoid fishes, which formed almost the whole vertebrate fauna. This unexpected discovery renders it almost certain, that numbers of other existing genera, of which we have found no traces, lived with these two through the whole secondary period; and we are thus obliged to assume as a probability, that any particular genus has lived through a long succession of geological ages. In estimating the importance of any peculiarities or anomalies in the geographical distribution of land shells as compared with the higher vertebrates, we shall, therefore, have to keep this possible, and even probable high antiquity, constantly in mind.