Colopisthus Richardson, 1902 is a rarely collected genus restricted in distribution to the tropical Atlantic Ocean with most species being known from the Caribbean region. Colopisthus species are highly endemic, none being known from more than a single island or a restricted mainland site. This monograph contains a redescription of the type species Colopisthus parvus Richardson, 1902, descriptions of four new species (Colopisthus canna, Colopisthus cavalier, Colopisthus ronrico, Colopisthus tresesquinas), a dichotomous key to the species, and a morphology-based phylogenetic analysis of the genus. Colopisthus parvus of Menzies and Glynn (1968) is shown to be a new species (C. ronrico), and C. parvus of Muller (1993) is also shown to be a new species (C. tresesquinas). As a result of the phylogenetic analysis, three species of Metacirolana Kussakin, 1979 (M. costaricensis, M. moortgati, M. sphaeromiformis) are assigned to a new genus, Aphantolana . Phylogenetic analysis indicates that Aphantolana and Colopisthus are sister groups. Within Colopisthus, C. cavalier and C. ronrico are a sister group to C. tresesquinas, C. parvus and C. canna . Biogeographic analysis suggests that Colopisthus and Aphantolana had their origins in the western Caribbean or pre-Pleistocene middle American seaway.
