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.