Crania, Ly. fig. 205. These are small brachiopodous shells, attached to other bodies; very frequently to the Echinites of the chalk. The free valve is commonly wanting, but I have found specimens dispersed in the rock. In many of the quarries in Kent and Sussex, the helmet Echinites bear groups of these shells. Ly. fig. 13.
Orbicula. This genus resembles Crania in form, the upper valve being like a limpet, whilst the attached valve is flat; it differs, however, from Crania in being horny and flexible, and is fixed to rocks on the bed of the sea, by a muscular pedicle passing out through a small fissure.
Species of Orbicula are found in strata of all ages, from the Lower Silurian to the Tertiary, and several are now living in tropical seas.
Obolus. Eichwald. In the Lower Silurian (Obolite grit) of Sweden and Russia, is a Lingula, with a hinge and a notch for the pedicle; it has not hitherto been found in Britain.
LINGULA. HIPPURITES.
Lingula. Ly. p. 353, fig. 412.—The Brachiopoda referred to this genus have a long peduncle, and their respiratory apparatus has no calcareous support; the recent species burrow in the sand, being usually inhabitants of shallow waters. The Lingulæ aære readily distinguished from the Terebratulæ by their imperforate, equivalved shells. One species is found in the Aymestry limestone, and several have been collected from the Mountain limestone, Oolite, and Shanklin sand.
With reference to the species of Brachiopoda, particularly of the Terebratulæ, which inhabit the depths of the ocean, Professor Owen observes, that "both the respiration and nutrition of animals, which exist beneath a pressure of from sixty to ninety fathoms of sea-water, are subjects suggestive of interesting reflections, and lead us to contemplate with less surprise the great strength and complexity of some of the minutest parts of the frame of these diminutive creatures. In the unbroken stillness which pervades those abysses, the existence of these animals must depend on their power of exciting a perpetual current around them, in order to dissipate the water laden with their effete particles, and to bring within the reach of their prehensile organs the animalcules adapted for their sustenance."
Hippurites. This genus belongs to a group of fossil shells whose characters are somewhat problematical, some conchologists referring them to the ordinary bivalves, and others to the Brachiopoda. Although Hippurites have not been discovered in the British strata, I am induced to notice them in this place, in consequence of their great abundance in the Cretaceous deposits of the South of France, and in the Oolite of the Pyrenees; and also to illustrate the nature of a nearly related genus, Sphærulites, of which one or more species occur in the Sussex Chalk.