The subkingdom Molluscoids has been richly represented by both the so-called "sea mosses" and brachiopods. The "sea mosses" form colonies of tiny mosslike tufts, resembling corals outwardly, though they are much more highly organized. They have been common from Ordovician time to the present, their carbonate of lime skeletons often having contributed to the building of limestone formations. Brachiopods always have two external shells or valves, in most cases working on a hinge, and also a pair of long, spiral-fringed arms associated with the soft part of the animal inside the shells. They differ from the other type of bivalve (e.g., clam, oyster) in that they are symmetrical with reference to a plane passed through the middle of the shells at right angles to the hinge line. They have rarely grown to be more than a few inches long. A few scant brachiopod remains are known from the late Proterozoic, but throughout known geologic time they reached their greatest development in the Paleozoic era, more especially in the Devonian period. Combining number of species and number of individuals, the brachiopods probably hold the record of all important groups of fossil animals, more than 7,000 species being known. Many layers of rock are filled with their shells (Plate 14). Since the close of the Paleozoic they have fallen off notably, and are now represented by relatively few small forms. From the standpoint of evolution it is interesting to note that in very early Paleozoic time the brachiopods were mostly small, of relatively simple organization, and their thin shells were not joined by hinges. Later they became larger and more complex and their thicker shells worked on hinges. Nearly all the Paleozoic forms had long, straight hinge lines, which made it difficult for their enemies to open them. Along with the change to narrower, curved hinge lines came the decline of the tribe. They have been of great value to the geologist in subdividing the geological column of strata into its many formations.
The Mollusks, which are more highly organized than the Molluscoids, have more or less distinctly developed heads and locomotive organs. Many thousands of species are now extinct, the classes of most geological importance being represented by clams, snails, and the pearly nautilus. Most of them have shells and gills for breathing. The members of the simplest group, well represented by the clam tribe, possess two similar shells working on hinges, so that in this regard they are much like brachiopods, but, unlike the latter, they are not symmetrical with reference to a plane at right angles to the hinge line. Cambrian strata contain the oldest known of the fossil forms where they are small, relatively thin-shelled, and rare. In marked contrast to the brachiopods these bivalves have rather steadily increased in numbers of species and individuals to the present time, now being represented by thousands of forms. During the Mesozoic era they greatly out-numbered the brachiopod bivalves and took on a more distinctly modern aspect, when the oyster tribe and closely related types were prominently developed. Culmination in size and thickness of shell seem to have been reached in early Cenozoic time, strata of that age in certain places, for example in Georgia and southern California, being filled with oyster shells 10 to 20 inches long and 4 to 6 inches thick! In addition to their gigantic size and thickness, many of the shells were fluted or ribbed, and so they represented an extreme type of defensive armor among the lower animals.