chap, viii.] ANTIQUITY OF LAND SHELLS. 169 



periods. In the Pliocene and Miocene formations, most of the 

 shells are very similar to living species, and some are quite iden- 

 tical. In the Eocene we meet with ordinary forms of the genera 

 Helix, Clausilia, Pupa, Bulimus, Glandina, Cyclostoma, Megalos- 

 toma, Planorbis, Paludina and Limncea, some resembling Euro- 

 pean 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, Mclania, 

 Paludina, Planorbis, and Limnma ; 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 Palaeozoic coal measures of Nova Scotia, 

 of two species of Helicidse, 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 unex- 

 pected 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 esti- 

 mating 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. 



We have now concluded our sketch of Tertiary Palaeontology 

 as a preparation for the intelligent study of the Geographical 

 Vol. I.— 13 



