The productus was furnished with slender hollow spines, which rose up from the surface of either valve, chiefly, however, about the hinge. In _P. spinosus_ they were long and stout, like thin rush stalks, while in the smaller species they rather resembled stiff bristles. The use of these spines is not very well made out. As most of the producti appear to have been free, that is, without any peduncle fixing them to the sea-bottom, it has been conjectured that the spines, by sinking deep into the mud, may have served the place of a peduncle to moor the shell. As regards size, the productus is very variable. You may gather some species in the young form, not larger than peas, while others may reward your search, having a breadth of six or eight inches (_P. giganteus_). But however much they may vary in dimensions, they usually remain pretty constant in their abundance, being among the most common fossils of the mountain limestone, and even of some limestones in the true Coal-measures;[37] and that must be a poor stratum indeed which cannot yield you a bagful of producti. [Footnote 37: See the table given below in Chap. X.] 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.