The geographical distribution of animals, with a study of the relations ...

—chap, viii.] ANTIQUITY OF LAND SHELLS. 169periods. In the Pliocene and Miocene formations, most of theshells are very similar to living species, and some are quite identical.In the Eocene we meet with ordinary forms of the generaHelix, Clausilia, Pupa, Bulimus, Glandina, Cyclostoma, Megalostoma,Planorbis, Paludina and Limncea, some resembling Europeanspecies, others more like tropical forms. A British Eocenespecies of Helix is still living in Texas ;and in the South of Franceare found species of the Brazilian sub-genera Megaspira andAnastoma. In the secondary formation no true land shells havebeen found, but fresh water shells are tolerably abundant, andalmost all are still of living forms. In the Wealden (LowerCretaceous) and Purbeck (Upper Oolite) are found Unio, Melania,Paludina, Planorbis, and Limncea ; while the last named genusoccurs even in the Lias.The notion that land shells were really not in existence duringthe secondary period is, however, proved to be erroneous by thestartling discovery, in the Palaeozoic coal measures of Nova Scotia,of two species of Helicidae, both of living genera Pupa vetusta,and Zonites priscus. They have been found in the hollow trunkof a Sigillaria, and in great quantities in a bed full of Stigmarianrootlets. The most minute examination detects no importantdifferences of form or of microscopic structure, between theseshells and living species of the same genera !These mollusca werethe contemporaries of Labyrinthodonts and strange Ganoid fishes,which formed almost the whole vertebrate fauna. This unexpecteddiscovery renders it almost certain, that numbers of otherexisting genera, of which we have found no traces, lived withthese two through the whole secondary period ; and we are thusobliged to assume as a probability, that any particular genus haslived through a long succession of geological ages. In estimatingthe importance of any peculiarities or anomalies in thegeographical distribution of land shells as compared with thehigher vertebrates, we shall, therefore, have to keep this possible,and even probable high antiquity, constantly in mind.We have now concluded our sketch of Tertiary Palaeontologyas a preparation for the intelligent study of the GeographicalVol. I.—13