In the lowest of these three horizons the shells are found, as already stated, in a thin bed of concretionary clay of dark grey colour, though associated with reddish beds. It contains Zonites priscus as well, though this is very rare, and there are a few valves of Cythere and shells of Naiadites as well as carbonaceous fragments, fronds of ferns, Trigonocarpa, etc. The Pupæ are mostly adult, but many very young shells also occur, as well as fragments of broken shells. The bed is evidently a layer of mud deposited in a pond or creek, or at the mouth of a small stream. In modern swamps multitudes of fresh-water shells occur in such places, and it is remarkable that in this case the only Gasteropods are land shells, and these very plentiful, though only in one bed about an inch in thickness. This would seem to imply an absence of fresh-water Pulmonifera. In the erect Sigillariæ of the second horizon the shells occur either in a sandy matrix, more or less darkened with vegetable matter, or in a carbonaceous mass composed mainly of vegetable débris. Except when crushed or flattened, the shells in these repositories are usually filled with brownish calcite. From this I infer that most of them were alive when imbedded, or at least that they contained the bodies of the animals; and it is not improbable that they sheltered themselves in the hollow trees, as is the habit of many similar animals in modern forests. Their residence in these trees, as well as the characters of their embryology, are illustrated by the occurrence of their mature ova. One of those, which I have considered worth figuring, has been broken in such a way as to show the embryo shell.
They may also have formed part of the food of the reptilian animals whose remains occur with them. In illustration of this I have elsewhere stated that I have found as many as eleven unbroken shells of Physa heterostropha in the stomach of a modern Menobranchus. I think it certain, however, that both the shells and the reptiles occurring in these trees must have been strictly terrestrial in their habits, as they could not have found admission to the erect trees unless the ground had been sufficiently dry to allow several feet of the imbedded hollow trunks to be free from water. In the highest of the three horizons the shells occurred in an erect tree, but without any other fossils, and they had apparently been washed in along with a greyish mud.[138]
[138] The discovery of the shells in this tree was made by Albert I. Hill, C.E. The tree is in Group XXVI. of Division 4 of my Joggins section. The original reptiliferous trees are in Group XV., and the lowest bed in Group VIII.
If we exclude the alleged Palæorbis referred to below, all the Palæozoic Pulmonifera hitherto found are American. Since, however, in the Carboniferous age, Batrachians, Arachnidans, Insects and Millipedes occur on both continents, it is not unlikely that ere long European species of land snails will be announced The species hitherto found in Eastern America are in every way strangely isolated. In the plant beds of St. John, about 9,000 feet in thickness, and in the coal formation of the South Joggins, more than 7,000 feet in thickness, no other Gasteropods occur, nor, I believe, do any occur in the beds holding land snails in Illinois. Nor, as already stated, are any of the aquatic Pulmonifera known in the Palæozoic. Thus, in so far as at present known, these Palæozoic snails are separated not only from any predecessors, if there were any, or successors, but from any contemporary animals allied to them.
It is probable that the land snails of the Erian and Carboniferous were neither numerous nor important members of the faunas of those periods. Had other species existed in any considerable numbers, there is no reason why they should not have been found in the erect trees, or in those shales which contain land plants. More especially would the discovery of any larger species, had they existed, been likely to have occurred. Further, what we know of the vegetation of the Palæozoic period would lead us to infer that it did not abound in those succulent and nutritious leaves and fruits which are most congenial to land snails. It is to be observed, however, that we know little as yet of the upland life of the Erian or Carboniferous. The animal life of the drier parts of the low country is indeed as yet very little known; and but for the revelations in this respect of the erect trees in one bed in the coal formation of Nova Scotia, our knowledge of the land snails and Millipedes, and also of an eminently terrestrial group of reptiles, the Microsauria, would have been much more imperfect than it is. We may hope for still further revelations of this kind, and in the meantime it would be premature to speculate as to the affinities of our little group of land snails with animals either their contemporaries or belonging to earlier or later formations, except to note the fact of the little change of form or structure in this type of life in that vast interval of time which separates the Erian period from the present day.
It may be proper to mention here the alleged Pulmonifera of the genus Palæorbis described by some German naturalists. These I believe to be worm tubes of the genus Spirorbis, and in fact to be nothing else than the common S. carbonarius or S. pusillus of the coal formation. The history of this error may be stated thus. The eminent palæobotanists Germar, Goeppert and Geinitz have referred the Spirorbis, so common in the Coal measures to the fungi, under the name Gyromyces, and in this they have been followed by other naturalists, though as long ago as 1868 I had shown that this little organism is not only a calcareous shell, attached by one side to vegetable matters and shells of mollusks, but that it has the microscopic structure characteristic of modern shells of this type.[139] More recently Van Beneden, Cænius, and Goldenberg, perceiving that the fossil is really a calcareous shell, but apparently unaware of the observations made in this country by myself and Mr. Lesquereux, have held the Spirorbis to be a pulmonate mollusk allied to Planorbis, and have supposed that its presence on fossil plants is confirmatory of this view, though the shells are attached by a flattened side to these plants, and are also found attached to shells of bivalves of the genus Naiadites. Mr. R. Etheridge, jun., of the Geological Survey of Great Britain, has summed up the evidence as to the true nature of these probably brackish-water shells, and has revised and added to the species, in a series of articles in the Geological Magazine of London, vol. viii.
[139] "Acadian Geology," 2nd edition, p. 205.
The erect trees of Coal Mine Point are rich in remains of Millipedes. The first of these (Xylobius Sigillariæ), which was the first known Palæozoic Myriapod, was described by me from specimens found in a tree extracted in 1852, and this, with a number of other remains subsequently found, was afterwards placed in the hands of Dr. Scudder, who has recognised in the material submitted to him eight species belonging to three genera (Xylobius, Archiulus, and Amynilyspes). These animals in all probability haunted these trees to feed on the decaying wood and other vegetable matter, and were undoubtedly themselves the prey of the Microsaurians. Though these were the earliest known, their discovery was followed by that of many other species in Europe and America, and some of them as old as the Devonian.[140]
[140] The two first-named genera from the erect trees, according to Scudder, belong to an extinct family of Millipedes, which he names Archiulidæ, and places with other Carboniferous genera in the order Archipolypoda. The third belongs to family Euphoberidæ. Proc. R. S. of London, 1892.
The only other remains of Air-breathers found in the erect trees belong to Scorpions, of which some fragments remain in such a state as to make it probable that they have been partially devoured by the imprisoned reptiles. No remains of any aquatic animals have been found in these trees. The Scorpions are referred by Scudder to three species belonging to two genera.[141]