Fig. 15. Kingena wacoensis, a common [Cretaceous] [brachiopod]. (a) [Dorsal] view. (b) [Lateral] view. (c) [Ventral] view.
[Phylum] Mollusca
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.
Plate 18
[PENNSYLVANIAN] BRACHIOPODS