Most species have a shell, but no operculum, although the group does also include several shell-less slugs.
The most common rocks observed in this form are sedimentary rocks (derived from what were formerly sediments), and extrusive igneous rocks (e.g., lavas, volcanic ash, and other formerly molten rocks extruded onto the Earth's surface).
The layers of rock are known as "strata", and the study of their succession is known as "stratigraphy".
A few principles were recognized and specified later.
An early summary of them is found in Charles Lyell's .
Pulmonates have a single atrium and kidney, and a concentrated, symmetrical, nervous system.
The mantle cavity is located on the right side of the body, and lacks gills, instead being converted into a vascularised lung.
There were also freehold estates not of inheritance, such as an estate for life.
Pulmonata, or "pulmonates", is an informal group (previously an order, and before that a subclass) of snails and slugs characterized by the ability to breathe air, by virtue of having a pallial lung instead of a gill, or gills.
To get to that point, there is also a historical discussion and description of non-radiometric dating methods.