Part 30

Ammonite—ammonoid cephalopod with complexly wrinkled suture pattern; member of subclass Ammonoidea.

Anterior—front or fore.

Anus—the terminal opening of the alimentary canal, through which waste matter is discarded from the body.

Aperture—the opening of shells, cells, etc.

Aragonite—calcium carbonate (CaCO₃) crystallizing in a different form than calcite. In shells it is chalky and opaque; is less stable than calcite.

Archeozoic—the oldest known geological era; early Precambrian time.

Articulated—joined by interlocking processes or by teeth and sockets.

Asymmetrical—without or lacking symmetry.

Bilateral—pertaining to the two halves of a body as symmetrical and mirror images of each other.

Binomial nomenclature—system of scientific nomenclature requiring two names: generic and trivial.

Blastoid—stalked echinoderm with bud-like calyx usually consisting of 13 plates; member of class Blastoidea.

Brachiopod—bivalved marine invertebrate; member of phylum Brachiopoda.

Brackish—a mixture of salt and fresh waters.

Burrow—a hole in the ground, rock, wood, etc., made by certain animals for shelter or while gathering food.

Calcareous—composed of, or containing, calcium carbonate; limy.

Calcite—calcium carbonate (CaCO₃) crystallizing in a different form than aragonite. In shells it is translucent and more stable than aragonite.

Cambrian—the first (oldest) period of the Paleozoic era.

Calyx—in corals the bowl-shaped depression in the upper part of the skeleton; in stalked echinoderms that part of the body which contains most of the soft parts.

Caprinid—a Cretaceous pelecypod that is typically coiled in the form of a ram’s horn.

Carapace—the hard protective covering that forms the exoskeleton of many invertebrates; in arthropods it is usually chitinous or calcaro-chitinous.

Carbonization—the process of fossilization whereby organic remains are reduced to carbon or coal.

Cast—the impression taken from a mold.

Cenozoic—the latest era of geologic time, following the Mesozoic era and extending to the present.

Cephalon—the head; in trilobites the anterior body segment forming the head.

Cephalopod—marine invertebrate with well-defined head and eyes and with tentacles around the mouth; member of class Cephalopoda, phylum Mollusca; includes squids, octopuses, pearly nautilus.

Ceratite—an ammonoid cephalopod with suture composed of rounded saddles and jagged lobes; member of subclass Ammonoidea.

Chert—a cryptocrystalline variety of silica; flint is a variety of chert.

Chitin—a horn-like substance, found in the hard parts of many animals, such as beetles, crabs, etc.