Most researchers have considered the calcar to be a unique and homologous structure within Chiroptera (e.g., the presence of this structure and its associated musculature has been cited as a synapomorphy supporting bat monophyly). However, we report that significant morphological variation exists between Microchiroptera and Megachiroptera. In microchiropterans, a calcified or cartilaginous element articulates directly with the calcaneal tuberosity of the ankle and projects into the uropatagium. In megachiropterans, a cartilaginous structure projects from the tendon of the gastrocnemius muscle into the uropatagium and has no articulation with the calcaneal tuberosity. Considerable variation also exists in the musculature associated with these structures. Phylogenetic interpretation of hindlimb morphology of extant and fossil taxa indicates that the calcar may not be homologous in all bats. We suggest retention of the term “calcar” for the microchiropteran structure and propose a new term, “uropatagial spur,” for the megachiropteran structure. The fossil bat Archaeopteropus transiens (Oligocene) has long been presumed to be a megachiropteran; however, this form has a microchiropteran-type calcar. Reconsideration of morphological evidence from this and previous studies indicates that Archaeopteropus is not a megachiropteran but, rather, a basal member of the microchiropteran lineage.