The _Gastropods_ of the carboniferous rocks in the British Islands embrace from twenty-five to thirty genera, with upwards of 200 species. Here, too, we can detect some forms that have not yet passed away. The _trochus_, so universally diffused over the globe at the present day, also lived in the palæozoic seas. Its companions, the _natica_, the _turritella_, and the _turbo_, likewise flourished in these ancient waters. Among the genera now extinct we may notice the _euomphalus_, with its whorls coiled in a flat discoidal form; and the _bellerophon_, with its simple coiled shell, resembling in general form the nautilus. The gastropods are numerously represented in our gardens and woods, by the various species of the snails, animals that have a most extensive distribution over the world, and number probably not much under two thousand species. [Illustration: Fig. 26.--Carboniferous Gastropods. 1. Euomphalus peatangulatus. 2. Pleurotomaria carinata (showing colour-bands).] For a long time it was matter of surprise that no such land shells had ever been detected in the carboniferous rocks. Trees and forests had been turned up by the hundred, but never a trace was found of any air-breathing creature. From this fact, and from the enormous amount of vegetable matter preserved, it was once hastily inferred that the atmosphere of that ancient period must have been uncongenial to air-breathers; that, in short, it was a dense heated medium of noxious carbonic-acid gas, wrapt round the earth like a vast mephitic exhalation, favourable in the highest degree to the growth of vegetation, yet deadly as the air of Avernus to all terrestrial animals. But this notion, like most other bold deductions from merely negative evidence, has had to be abandoned, for traces of air-breathers have at last been found. Among these, not the least interesting is the shell of a _pupa_, a sort of land-snail, which Sir Charles Lyell detected, along with the bones of a small reptile, embedded in the heart of an upright sigillaria stem in the carboniferous rocks of Nova Scotia. Small as was the organism, the evidence furnished by it proved scarcely less valuable than if it had been a large mammal that might have afforded material for weeks of study. The similarity of the shell to existing forms, showed that the ancient carboniferous forests had at least one race of air-breathing creatures among their foliage, and that the atmosphere of the period could have differed in no material point from that of the present day, for as the snails breathe by lungs, and require, consequently, a continual supply of oxygen to support respiration, they could not have existed in an atmosphere charged with carbonic acid. [Illustration: Fig 27.--Carboniferous Cephalopods. 1. Nautilus Koninckii. 2. Goniatites crenistria. 3. Orthoceras laterale (fragment).]