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It seems that comparatively few of the gasteropods and lamellibranchiate bivalves of North America can be identified specifically with European fossils, while no less than two-fifths of the brachiopoda, of which my collection chiefly consisted, are the same. The predominance of bivalve mollusca of this peculiar class has caused the Silurian period to be sometimes styled "the age of brachiopods."
The Brachiopods have passed into entirely new and more advanced species in the many advances and retreats of the shores, but the Molluscs show more interesting progress. The commanding group from the start is that of the Molluscs which have "kept their head," the Cephalopods, and their large shells show a most instructive evolution.
The annexed wood-cut represents one of these Brachiopods, which form a very characteristic type of the Silurian deposits. The square cut of the upper edge, where the two valves meet along the back and are united by a hinge, is altogether old-fashioned, and unknown among our modern Bivalves.
Notwithstanding the antiquity of this Russian formation, it should be stated that both of these genera of brachiopods have been also found in the Upper Silurian of England, i.e. In the Wenlock limestone. Among the green grains of the sandy strata above-mentioned, Professor Ehrenberg announced in 1854 his discovery of remains of foraminifera. Clinton Group: Beds of Passage, Llandovery Group.
In the most primitive order that of the INARTICULATE brachiopods the two valves are held together only by muscles of the animal, and the shell is horny or is composed of phosphate of lime. The DISCINA, which began in the Algonkian, is of this type, as is also the LINGULELLA of the Cambrian.
The two valves of the brachiopod shell are unequal in size, and in each valve a line drawn from the beak to the base divides the valve into two equal parts. It may thus be told from the pelecypod mollusk, such as the clam, whose two valves are not far from equal in size, each being divided into unequal parts by a line dropped from the beak. Brachiopods include two orders.
In its embryological development it passes through the long-tailed stage; connecting links in the Mesozoic also indicate that the younger type is the offshoot of the older. Insects evolve along diverse lines, giving rise to beetles, ants, bees, and flies. Brachiopods have dwindled greatly in the number of their species, while mollusks have correspondingly increased.
WORMS. Trails and burrows of worms have been left on the sea beaches and mud flats of all geological times from the Algonkian to the present. BRACHIOPODS. These soft-bodied animals, with bivalve shells and two interior armlike processes which served for breathing, appeared in the Algonkian, and had now become very abundant.
At many places corals formed thin reefs, as at Louisville, Kentucky, where the hardness of the reef rock is one of the causes of the Falls of the Ohio. Sponges, echinoderms, brachiopods, and mollusks were abundant. The cephalopods take a new departure.
In the Silurian ages, invertebrates brachiopods and crinoids and cephalopods were the dominant types. But very early no one knows just when there came fishes of many strange forms, some of the early ones enclosed in turtle-like shells.
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