[Illustration: 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] [Footnote 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. [Illustration: Fig. 25.--Carboniferous Lamellibranchs. 1. Aviculopecten sublobatus (showing colour-bands). 2. Conocardium aliformis.]