CHAP, VIII. ] ANTIQUITY OF LAND SHELLS, 169 
periods. Inthe 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 Limnea, 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, Melania, 
Paludina, Planorbis, and Limnea ; 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 disvovery, in the Paleozoic coal measures of Nova Scotia, 
of two species of Helicidee, 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 Paleontology 
as a preparation for the intelligent study of the Geographical 
Vou L—1s 
