The productus no longer ranks among living forms. It began during the times of the Upper Silurian system, lived all through the Old Red Sandstone, and attained its maximum of development in the seas of the Lower Carboniferous group. As the coal forests began to flourish, the productus seems to have waned; but it is still sometimes found in considerable numbers in the ironstones and limestones intercalated among the coal seams of northern England and central Scotland. In the period which succeeded the coal, that, namely, of the Permian, it seems to have died out altogether, at least no trace of its remains have as yet been detected in strata of a later age. But whilst it lived, the productus must have enjoyed a wide range of climate, for its valves have been found by thousands both in the old world and in the new. I have seen several that were brought from the hills of China, and they occur likewise in Thibet. Specimens have been brought, too, from the warm plains of Australia, and from the snows of Spitzbergen.
In looking over the fossils that lie grouped along beds of the mountain limestone, there are two forms that we find almost invariably side by side—the productus and the spirifer. They seem to have begun life together, or rather, perhaps, the spirifer is somewhat the older brother. They voyaged through the same seas, and anchored themselves to the same ocean-bed, sometimes among mud and ooze, and often among bowers of corals and stone-lilies. They visited together the most distant parts of the world, from China to Chili, and from Hudson's Bay to New Zealand. I have sometimes laid open fragments of limestone where they lay thickly clustered as though they had ended a life of friendship by dying very lovingly together. But after all the varieties of the productus had died out, some species of the spirifer still lived on, and it was not until the period of the lias that they finally disappeared. I remember meeting with one of these latest spirifers in the course of a ramble in early morning along the shores of Pabba, one of the lone sea-girt islands of the Hebrides, where the Scottish secondary rocks are represented. The beach was formed of low shelving reefs of a dark-brown micaceous shale, richly charged with the characteristic fossils of the Lias—ammonites, belemnites, gryphææ, pectines, &c. In the course of the walk I came to a lighter coloured band, with many reddish-brown nodules of ironstone, but with no observable fossils. A search, however, of a few minutes disclosed a weathered specimen, near which a limpet had made good its resting-place; and this solitary specimen proved to be one of the last lingering spirifers (S. Walcottii). The form struck me at once as a familiar one, and recalled the fossils of the mountain limestone. It may seem a puerile fancy, but to one who had lately been working among palæozoic rocks, and remembered the history of the spirifer, there was something suggestive in the loneliness of the specimen. With the exception of one or two other organisms (as rhynconella), it was by far the most ancient form of the deposit. Its family had come into the world thousands of years before that of the large pinnæ that lay among the neighbouring shales, and perhaps millions of years before that of the gracefully curved ammonites. But the family was nearly extinct when these shales were being thrown down as sandy mud, and this wasted specimen, worn by the dash of the waves, seemed in its solitariness no inapt representative of an ancient genus that was passing away.
The spirifer received its name from the two highly developed spiral processes in the interior of the shell attached to the dorsal valve. They were hard, like the substance of the shell, and sprang from near the hinge, each diverging outwards to near the border of the valve. They resembled two cork-screws, but the loops were much closer together. These coiled calcareous wires almost filled the hollow of the shell ([Fig. 23]), and ample support was thus afforded to the filamentous arms. In recent brachiopods, these arms do not always strictly follow the course of the calcareous loops. Among palæozoic genera the case may have been similar, so that the complex calcareous coil of the spirifer may not perhaps indicate a corresponding complexity of the arms. But none of the few recent forms exhibit anything like the coiled processes of the spirifer.
The Carboniferous system of Great Britain and Ireland is stated to have yielded between fifty and sixty species of spirifers. Of course, in such a long list the gradations are sometimes very nice, and to an ordinary eye imperceptible, but there exist many marked differences notwithstanding. The general type of the spirifers is tolerably well defined. They had both valves arched outwards, not concavo-convex as in the productus. Their hinge-line, like that of the latter shell, ran in a straight line, and their dorsal valve was raised along its centre from hinge to outer margin, into a prominent ridge, while in the ventral valve there was a furrow exactly to correspond. Most of the species were traversed by sharp ribs radiating from the centre of the hinge-line like those on the surface of the common cockle. But some were quite smooth, retaining only the high lobe in the centre, such as S. glaber. In a noble specimen figured by M'Coy[38] under the name of S. princeps, the valves are covered with broad plaits that sweep gracefully outward from the centre of the hinge-line.
[38] Carb. Limest. Foss. of Ireland, pl. 21, fig. 1.
Fig. 23.—Spirifer hystericus. b, Interior of the same, showing the arrangement of the spiral arms.
The spirifers vary more in form than in external ornament. Some are triangular, others nearly semicircular, others long and attenuated. In some species (as the S. glaber), the central ridge is very prominent, taking up about a third of the entire area of the shell, and thus giving it a trilobed appearance. In others (as S. symmetricus) it is less marked, and bears a minor furrow down its centre; while in yet a third class (as in some specimens of S. trigonalis) the median fold scarcely rises above the ribs that are ranged on each side.
These old shells probably anchored themselves to the sea-bottom by means of a thin peduncle, and lived by the vigorous action of those complex fringed arms, whose screw-like skeleton still occasionally remains, and which conveyed to the mouth the animal substances that served as food.
