The phylum Mollusca encompasses a large group of aquatic
(water-dwelling) and terrestrial (land-dwelling) invertebrates which
includes such familiar forms as the snails, clams, oysters, squids, and
octopuses. Most mollusks possess a calcareous shell that serves as an
exoskeleton, and these hard parts are well adapted for preservation as
fossils. However, some mollusks (the slugs) have no shells, and others
(the squids) have an internal calcareous shell. Because of their
relative abundance and great variety, mollusks are particularly useful
fossils. Moreover, the remains of certain mollusks, such as the oysters,
are important rock builders.
The phylum Mollusca has been divided into five classes:
1. _Amphineura_—the chitons or sea-mice; shell composed of eight valves
or plates; not a common fossil. Ordovician to Recent.
2. _Scaphopoda_—the tusk-shells; shell composed of a single tusk-like
valve; generally not a common fossil but locally abundant in certain
Cenozoic formations. Devonian to Recent.
3. _Gastropoda_—the snails and slugs; slugs are without shells, snails
have a single-valved shell which is typically coiled; common fossils in
Paleozoic, Mesozoic, and Cenozoic rocks. Cambrian to Recent.
4. _Pelecypoda_—clams, mussels, oysters, scallops; shells composed of
two valves, usually, but not always, of equal size; common fossils,
especially in Mesozoic and Cenozoic rocks. Cambrian to Recent.
5. _Cephalopoda_—squids, octopuses, the pearly nautilus, and the
ammonoids (extinct); shell of one valve, usually coiled and partitioned
by septa; valuable fossils, especially in Paleozoic and Mesozoic rocks.
?Cambrian, Ordovician to Recent.
[Illustration: Plate 18
PENNSYLVANIAN BRACHIOPODS]
MARGINIFERA × 1
AMBOCOELIA × 1
SQUAMULARIA × 1
DERBYA × 1
MESOLOBUS × 1
CHONETES × 1
LINOPRODUCTUS × 1
PUNCTOSPIRIFER × 1
COMPOSITA SUBTILITA × 1
NEOSPIRIFER × 1
[Illustration: Plate 19
PENNSYLVANIAN BRACHIOPODS]
JURESANIA × 1
SPIRIFER ROCKYMONTANUS × 1
NEOSPIRIFER CAMERATUS × 1
Of these five classes, only the Gastropoda, Pelecypoda, and Cephalopoda
are discussed herein.