O octopus: 66, 78 Odessa College: 27 operculum: 59 _Ophiuroidea_: 82 ophiuroids: 82 _opima, “Marginifera”_: 12, 13 order, taxonomic: 22 Ordovician— derivation and pronunciation: 34 fossils: 40 graptolites: 86 of El Paso region, Llano, Marathon, Solitario, and Van Horn uplifts: 40 ornamentation, brachiopod: 54 gastropod: 59 pelecypod: 66 Ornithischia: 97 ornithischians: 90, 97, 98, 100 Ornithopoda: 97 ornithopods: 90, 97, 99 _Orthoceras_: 66, 76 _Orthoyoldia_: 73 ossicles. _See_ sclerites. _Osteichthyes_: 87 Ostracoda: 80 ostracoderms: 87 ostracodes: 78, 79, 80 _Ostrea carinata_: 71 _lisbonensis_: 72 _quadriplicata_: 71 _sellaeformis_: 72 _Oxytropidoceras_: 77 oysters: 56, 59 P _Pachecoa_: 72 _Pachymya_: 71 paleobotany: 4 paleobotanists: 44 paleontology— definition: 4 divisions of: 4 history of: 3 invertebrate: 4 vertebrate: 4 _Paleoscincus_: 90, 97, 99, 100 Paleozoic— derivation and pronunciation: 33 periods of: 34 rocks of Texas: 40-42 Palo Pinto County: 20 Paluxy Creek: 14, 15 Pan American College: 27 Pantodonta: 102 pantodonts: 102 _Parasaurolophus_: 90 _Parasmilia_: 53 Parker County: 20 Pawpaw formation: 11 pearly nautilus: 66 morphology: 75 Pecos County: 42 Pecos River valley: 35 _Pecten_: 59, 68, 74 pedicle: 54, 56 foramen: 54, 56 valve, brachiopod: 54 Pelycosaurs: 89, 92 Pelecypoda: 56, 59, 65-66 pelecypods: 26, 29, 30, 56, 59, 60, 66 Cretaceous: 59, 66, 68-_71_ dentition: 66 morphology: 59, 60, 65, 66 ornamentation: 66 Pennsylvanian: 66, 67 teeth: 60, 65, 66 Tertiary: 72, 73, 74 Pelmatozoa: 80, 81 Pennsylvanian— derivation and pronunciation: 34 fossils: 41 brachiopods: 57, 58 bryozoans: 55 cephalopods: 66, 76 corals: 52 crinoids: 41, 82, 83 fusulinids: 47, 49 gastropods: 61, 62 nautiloids: 76 pelecypods: 66, 67 plants: 46, 47, 48 shark teeth: 87 sponges: 50 of Diablo and Hueco Mountains, Llano and Marathon uplifts, and north-central Texas: 41 _Pentaceros_: 83 _Pentagonaster_: 83 _Pentremites_: 81 period, geologic: 33 periostracum: 60 Perissodactyla: 104 perissodactyls: 103, 104, 105 Permian— derivation and pronunciation: 34 fossils: 41 amphibian: 89, 92 brachiopods: 12, 13 cotylosaurs: 89, 92 pelycosaurs: 89, 92 of Glass Mountains: 11, 12, 35 _permiana, Prorichthofenia_: 12, 13 permineralized bone: 9 petroleum geologist: 4 _Phaneroceras_: 76 _Phobosuchus_: 95, 96 _Pholadomya_: 71, 72 phyla: 22 _Phyllograptus_: 86 phylum: 22