Twenty species of bivalves are described from the Ixtaltepec Formation, in Nochixtlán, México. Representatives of the genera Parallelodon, Myalinella, Septimyalina, Leptodesma, Pterinopectinella, Aviculopecten, Limipecten, Streblopteria, Euchondria, Schizodus, Permophorus, Astartella, Edmondia, and Ectogrammysia are present. These species, and those of nuculoid bivalves previously described from the same formation, include infaunal and epifaunal forms that lived in shallow marine environments from Morrowan to Desmoinesian ages. Pennsylvanian bivalves from Nochixtlán are related to the Midcontinent and Appalachian faunas, which indicate that there was an oceanic connection between these areas in a latitudinal belt near the equator that partially covered southeastern Mexico.

Five species of nuculoid bivalves are described from Pennsylvanian outcrops of the Ixtaltepec Formation, Oaxaca, Mexico: Nuculopsis sp., Palaeoneilo sinuosus new species, Paleyoldia sp., Phestia bellistriata, and Phestia sp. have been collected in clastic rocks and in association with other marine invertebrate groups, which represent a peri-reefal environment.

Twenty-three species of Campanian, Maastrichtian, Paleocene, and Eocene freshwater gastropods from northeastern Mexico are described, including eight new species:Viviparus mcbridei, Pachychilus(Pachychiloides)lawtoni, Melanoides(Melanoides)yolandae, Melanoides(Melanoides)wollebeni, Physa cepedaensis, Mesolanistes magnus, Mesolanistes murrayi, andGyraulus zoltani.Specimens were collected from fine-grained, green sandstone and mudstone, red mudstone, and as hematized remains in fine light-brown sandstones belonging to the following stratigraphic units of northeastern Mexico: Cerro del Pueblo (Campanian, Parras Basin), Olmos (Maastrichtian, Sabinas Basin), Las Encinas (Paleocene, Parras Basin), and Carroza (Eocene, La Popa Basin) Formations. All except two of the genera (?PyrguliferaandMesolanistes) have recent representatives. Review of habitats of living species of the six extant genera and interpretations of sample lithologies support the inference of dominantly freshwater paleoenvironments. Only one Maastrichtian locality is interpreted confidently to have had a brackish water influence.

La función e importancia de las colecciones científicas radica en los objetos de naturaleza única que contienen, materia prima para generar y validar conocimiento científico. Las colecciones paleontológicas preservan, conservan y mantienen disponible los fósiles y su información. Con ello, protegen el patrimonio más preciado de la sociedad: el conocimiento científico de al menos una parte de la naturaleza y sus beneficios sociales. Palabras clave: Colecciones científicas, colecciones paleontológicas, fósiles.

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