The little visceral cavity contains the complex groups of muscles for opening and closing the valves, a simple stomach, a large granular liver, a short intestine, two hearts, and the centre of the nervous system. Without going into the details of these various structures, the reader will see that the brachiopoda are really a highly organized tribe; and I am thus particular in the enumeration, partly that he may the better understand the mechanism of the carboniferous shells of that type, and partly that he may mark how the oldest forms of life, those that meet us on the very threshold of animated existence, were not low in organization, but possessed an anatomy as complex as it was beautiful. Who that has ever wielded an enthusiastic hammer among the richly fossiliferous beds of the mountain limestone, does not remember with delight the hosts of delicately fluted shells that the labour of an hour could pile up before him? There was the striated productus, with its slim spines scattered over the stone. There, too, lay the spirifer with its broader plications, its toothed margin, and its deeply indented valve. Less common, and so more highly prized, was the slimly-ribbed rhynconella, with its sharp, prominent beak, or perhaps the smooth, thin terebratula, with its colour-bands not yet effaced. These were pleasant hours, and their memory must dwell gratefully among the recollections of one whose avocations immure him throughout well-nigh the livelong year amid the din and dust of town--the _fumum et opes strepitumque Romæ_. [Illustration: Fig. 22.--Productus giganteus.] In the _productus_ the dorsal valve is sometimes quite flat, while the ventral is prominently arched, and the shell resembles a little cup with a flat plate of the same diameter placed over it. Usually, however, both the valves are concavo-convex, or arched in the same direction like two saucers placed within each other. The exterior surface of each valve is differently ornamented in the various species. A very common style of sculpturing is by a set of fine hair-like longitudinal ribs, diverging more or less regularly from the hinge line to the outer margin. In some species these ribs are wider, and are furnished with little prominent scars. In others (as _P. punctatus_) a set of semicircular ridges runs round the shell, narrowing as they converge from the outer lips to the centre of the hinge line, and bearing each an irregular row of small scars or tubercules. Some of the species are very irregularly ornamented into a sort of wrinkled surface, in which the striæ seem, as it were, thrown over the valves in bundles at random.