A survey of cave and spring fauna in Oklahoma resulted in the discovery of Caecidotea mackini, new species, which is described from specimens collected in Long's Cave, Delaware County. Within the hobbsi group of the genus Caecidotea, C. mackini belongs to a subset of nine species termed the tridentata assemblage. All nine are subterranean, but several retain vestigial eyes or pigmentation. Correlated with zoogeographic and climatic evidence, the invasion of groundwaters by an epigean progenitor during the middle to late Tertiary is suggested. Besides C. mackini, new records for C. macropropoda, C. acuticarpa, C. stiladactyla, C. steevesi, C. ancyla, C. antricola, C. adenta, and C. simulator expand the known ranges of these cryptic species. With the recognition of two patterns of fourth pleopod morphology in C. acuticarpa, the species specificity of this character has become questionable. Although previously used to differentiate C. simulator and C. steevesi, further splitting or synonymy based on this pleopod anatomy is reserved until a better understanding of its differentiation is achieved.