In this study nearly 50 species of diatoms new to science, collected from lake sediments across the western USA, are presented for the first time. A total of thirty-eight raphid species are described (across eighteen genera), while one non-raphid pennate and nine non-pennate species are also described from these collections. Observations with light and scanning electron microscopy are offered for each taxon. Nomenclatural and morphological details for previously known species encountered in the author's examination of these sediments are provided and indicate their distribution across the western USA. 80 plates, a taxon index and a comprehensive reference list round up this volume and make it a valuable source of information for biologists and palaeontologists working not only on American but also on diatoms worldwide.