Ascending from the Devonian to the Carboniferous, we at once find ourselves in the midst of air-breathers of various types. Here are Myriapods, insects of several orders, Spiders, Scorpions, Land-snails, and Batrachian reptiles, and these of many species, and found in many localities widely separated. We can thus people those dark, luxuriant forests, to which we owe our most valuable beds of coal, with many forms of life; and as most of these belong to tribes likely to multiply abundantly where food was plentiful, we can imagine multitudes of Snails and Millepedes feeding on succulent or decaying vegetable matter, swarms of insects flitting through the air in the sunnier spots, while their larvæ luxuriated in decaying masses of leaves or wood, or peopled the pools and streams. In like manner, in imagination we can render these old woods vocal with the trill of crickets and with the piping or booming of smaller and larger Batrachians. Let us now, in accordance with our plan, inquire as to the nature of these early air-breathers and the fortunes of their families in the geological history.
Fig. 124.—Land-snail (Pupa vetusta, Dawson). From the Coal-formation.
a, Natural size. b, Magnified. c, Apex. d, Sculpture. Enlarged.
Fig. 125.—Land-snail (Zonites (Conulus) priscus, Carpenter). From the Coal-formation.
a, Shell. Enlarged; the line below shows the natural size. b, Sculpture. Enlarged.
The Land-snails known as yet in the Carboniferous are limited to five or six species, belonging to four genera, all American and related to existing American forms. The two earliest known are represented in [Figs. 124 and 125].[44] One of them is a Pupa, or elongated Land-snail, so similar to modern forms that it does not merit a generic distinction, and is indeed very near to some existing West Indian species. The other is in like manner a member of the modern genus Zonites. These are from the Coal-formation of Nova Scotia, and the Pupa must have been very abundant, as it has been found in considerable numbers in a layer of shale, and in the stumps of erect trees, in beds separated from each other by a thickness of 2,000 feet of strata. The Zonites is much more rare. A second Pupa is found in Nova Scotia, and two species occur in the Coal-field of Illinois. One of these is a Pupa still smaller than P. vetusta, and, like some modern species, with a tooth-like process on the inner lip. The other has been placed in a new genus,[45] but is very near to some of the smaller American Snails still living. Its most special character is a plate extending from the inner lip over half the aperture, a contrivance for protection still seen in some modern forms. Thus the Land-snails come on the stage in at least three generic forms, similar to those which still live, but all of small size, indicating perhaps that the conditions were less favourable for such creatures than those of the temperate and warmer climates at present. It may seem a small step in advance for Sea-snails to lose their gills and to become Land-snails, and this without any elevation of their general structure; but it must be borne in mind that we have here not only the dropping of the gills for an air-sac, but profound changes in teeth, mucous glands, shell, and other particulars, to fit them for new food and new habits. It is also singular that the Land-snails at once appear instead of the intermediate forms of the air-breathing fresh-water snails. These last may, however, yet be found.