This paper reviews the status of 39 Anacroneuria species reported for Mexico, the adjacent areas of Mesoamerica (Belize, Guatemala, Honduras, Nicaragua), and the United States. Sixteen species are described as new (A. baumanni, A. brailovskyi, A. buenoi, A. contrerasi, A. izapa, A. mixteca, A. olmec, A. pareja, A. quetzalcoatl, A. ratcliffei, A. senahu, A. shepardi, A. sonora, A. wellsi, A. zaculeu, and A. zaga). Anacroneuria sulana Needham and Broughton is removed from synonymy with A. annulicauda (Pictet) and classified as a synonym of A. lineata (Navas). Anacroneuria proxima Klapálek, A. crenulata Jewett, and A. comanche Stark and Baumann are classified as synonyms of A. litura (Pictet). Identification keys are presented for males in the region, and the nymphs of 2 species, A. baumanni and A. quadriloba, are described.

This paper presents descriptions, diagnoses, figures, and keys for 12 western North American species of the Holoarctic genus Isocapnia Banks. Complete known distributional records and a cladistic analysis are included for the Nearctic species. Isocapnia fraseri and I. missourii are synonymized with I. integra, and 3 new species are described: I. eichlini n. sp., I. palousa n. sp., and I. rickeri n. sp. Representatives of 5 of the 7 known Palearctic species of Isocapnia were examined and are compared with the Nearctic species. Type localities and distributional data are listed for all 7 Palearctic species.

This article reviews the capniid genus Paracapnia Hanson and describes a new species, P. boris, from males and females collected in northern California, provisionally assigned to the genus. Paracapnia oswegaptera (Jewett) is placed in synonymy with P. ensicala (Jewett), and the first description is provided for the male of P. disala (Jewett). Keys are presented for recognized Palearctic and Nearctic members of the genus. Distribution data are given for specimens examined and for those in the published literature.

Field collections at more than 2900 sites and the examination of many museum collections and literature allowed me to map the historical and current distribution of several freshwater molluscan faunal groups in the Intermountain region of the United States (Great Basin, Colorado River drainage basin, and upper Snake River sub-basin). Historical and current records show that Margaritifera falcata, Anodonta californiensis, and Ferrissia rivularis have drainage-specific distributions, while Valvata utahensis has a specific drainage pattern, and V. californica (new combination) has a dispersed pattern. Shell morphometric data of Valvata and Ferrissia show extensive shell variation between and within populations. Current surveys show that these molluscan populations have been reduced since the colonization by European descendants over the last 150 years. Margaritifera falcata was found to be extirpated from eastern California, Nevada, and Utah and was common in only 1 stream. Anodonta californiensis populations of 10 or more individuals occurred in only 2 of 13 drainages, as well as in 1 isolated spring. Valvata californica was extirpated in 7 of 10 lakes. Ferrissia rivularis was very rare in 6 of 12 drainages. Range declines among these fauna are thought to be related to alterations of habitat caused by grazing, irrigation, and urbanization, as well as the intensive management of sport fish in these waters.