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