Fig. 24.—Terebratula hastata.
I shall refer to but one other brachiopod of the carboniferous rocks, interesting both as one of the forms of life still living in our seas, and as exhibiting, after the lapse of such a vast interval, the form of the coloured bands which adorned it when alive. It is called Terebratula hastata; a slim delicate shell like its representatives of the present day, narrow at the beak, and bulging out towards the outer margin, which is slightly curved. The surface is smooth, and in the older specimens has numerous concentric layers of growth, especially marked near the margin. The stripes of colour radiate from the beak, outwards, and though the tint which once brightened them is no longer visible, it may be that the vessel of the little terebratula, which lay anchored perhaps fifty fathoms down, was well-nigh as gaily decked as a felucca of the Levant. But the existence of these colour-bands is not merely interesting; the geologist can turn it to account in investigating the physical conditions of an ancient ocean. The late Professor Edward Forbes, after a careful series of investigations in the Mediterranean, brought to light the fact, that below a depth of fifty fathoms shells are but dimly coloured, and hence he inferred, from the numerous coloured shells of the carboniferous limestone, that the ocean in which they lived was not much more than fifty fathoms deep.[39]
[39] Similar coloured bands are found even in the Lower Silurian, e.g., on turbo rupestris (Murchison's Siluria, p. 194), while on many of the carboniferous gastropods and lamellibranchiate bivalves, they are of frequent occurrence.
The lamellibranchiate bivalve shells of the British Carboniferous system, so far as yet discovered, number about 300 species, belonging to genera some of which are still familiar to us. There were the pectens or scallops, the pinnas with their beards of byssus, the cardiums or cockles, the mytili and modiolæ or mussels, all sea-shells. Then among the fresh-water bivalves we can detect several species of the unio or river mussel, that perhaps displayed valves as silvery in their lining as those of our own pearl-mussels. But with these well-known forms there co-existed some that no longer survive. Such was the conocardium, a curious form that looks like a cardium cut through the middle, with a long slender tube added to the dismembered side ([Fig. 25]). The aviculopecten, a shell allied to our common scallop, and sometimes showing still its colour-bands ([Fig. 25]), and the cardinia or anthracosia, a small bivalve that abounds in the shales and ironstones of our coal-fields, along with nautili, producti, and conulariæ at Coalbrook Dale, and with a thin leaf-like lingula at Borrowstounness.
Fig. 25.—Carboniferous Lamellibranchs.
1. Aviculopecten sublobatus (showing colour-bands). 2. Conocardium aliformis.
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.
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.