c. Plane of symmetry at right angles to hinge line;

inequivalved; not “scallop-like” and without “ears.” Larger valve commonly has an opening in beak. Common in Paleozoic rocks Brachiopod 2. Fossil tapering, cylindrical, cone-shaped: a. Fossil with internal longitudinal, radial septa or partitions; cone-shaped Coral b. Shell with internal transverse partitions or sutures; tapering at one end Cephalopod c. Shell without internal septa or partitions. (1) Shell large, heavy; usually with external longitudinal ribs. Occur only in Cretaceous rocks Rudistid (2) Shell small (usually less than 2 inches), tusk-shaped, open at both ends. Rare in Paleozoic and Mesozoic rocks Scaphopod 3. Fossil heart-shaped, domed or flattened; radial star pattern on top Echinoid 4. Fossil segmented: a. Fossil divided into 3 lobes; may be curled up. Not found in Mesozoic or Cenozoic rocks Trilobite b. Fossil flattened or elongate; resembles shrimp, crab, or crayfish Crustacean III. Fossils displaying no apparent symmetry A. Shell without transverse internal partitions or sutures: 1. Shell coiled like ram’s horn, low spired, opening of shell very large; surface has concentric ridges. Shell has two valves; smaller, flattened valve not often found. In Texas found only in Cretaceous rocks Pelecypod (Note: Some Paleozoic gastropods, “2,” closely resemble larger valve of these pelecypods) 2. Shell tightly coiled; most have higher spire than “1.” Opening of shell smaller than “1”; shell not as rough as “1” and has only one valve Gastropod B. Coiled fossils; coiling not in one plane: 1. Shell with transverse internal partitions or sutures: a. Partitions always smooth; thick shelled; loosely and irregularly coiled, usually in large compact masses of many individual shells. Occur only in Cretaceous rocks Caprinid b. Partitions (sutures) usually wrinkled; relatively thin shelled; mostly regularly and tightly coiled; occur as separate individual specimens Cephalopod 2. Shell without transverse internal partitions or sutures Gastropod 3. Solid spiral ridge around central axis; resembles a corkscrew Bryozoa C. Uncoiled fossils: 1. Fossil resembles a narrow saw blade; typically found as thin film of carbon. Not found in Mesozoic or Cenozoic rocks Graptolite 2. Fossil irregularly cone-shaped; longitudinal radial partitions or septa Coral 3. Shell resembles a clam or oyster shell but valve or shell not symmetrical Pelecypod (mostly oysters) 4. Branching twig-like fossils: a. Fossils covered with minute pores or openings Bryozoa b. Fossils with evenly distributed, relatively large openings with longitudinal radial partitions or septa Colonial coral 5. Lace-like fossils; occur as thin sheets or films Bryozoa 6. Fossils composed of radiating masses of polygonal or circular tubes containing radial septa Colonial coral 7. Irregular fossils; typically cylindrical with rough surface: a. Fossil has large axial opening and thick wall; usually has external longitudinal ribs. Occurs only in Cretaceous rocks Rudistid b. Fossil solid with no large axial opening; surface with small pits or pores (fewer than in Bryozoa). In Texas, occurs most commonly in Pennsylvanian and Permian rocks Sponge

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A. Tapering, cylindrical cone-shaped fossils 1. Cone-shaped with longitudinal partitions or septa Coral 2. Fossils with septa or sutures; tapering at one end Cephalopod 3. Shell without internal partitions or sutures a. Shell large heavy, external longitudinal ribs. Cretaceous only Rudistid b. Shell small, tusk-shaped open at both ends. Rare in Paleozoic and Mesozoic Scaphopod B. Disc or dome-shaped fossils 1. Star pattern on top Echinoid 2. Subconical small pits or pores on top Bryozoan 3. Small disc-shaped (less than ½ inch) Orbitoid Foraminifera 4. Disc-shaped or button-like, with longitudinal partitions or septa Coral C. Fossils composed of segments or plates 1. Circular discs or chambers; when united form cylinder a. Tapered shell Cephalopod b. Not tapered, segments small of uniform thickness, hole in center Crinoid Stem 2. Fossil composed of many-sided plates a. Bud-shaped, 13 wedge-shaped plates Blastoid b. Cup-shaped, many curved plates branching arms Crinoid

A. Fossil coiled in a single plane 1. Shell divided by internal transverse partitions or sutures Cephalopod 2. Shell without internal partitions or sutures Gastropod 3. Shell small, spindle-shaped; resembles wheat grain. Pennsylvanian and Permian Foraminifera fusulinid B. Fossil not coiled 1. Shells or valves similar to clams a. Plane of symmetry parallel to hinge; equivalved Pelecypod b. Plane of symmetry almost at right angles to hinge; strongly ribbed; “Scallop-like” with “ears”, inequivalved Pelecypod