Abstract. The genus Bitheca is described for a distinctive clade of small Limosininae whose range extends from Canada south to Ecuador. The following twelve species are described as new: agarica, boleta, caballa, dispar, ejuncida, fundata, grossa, horrida, involuta, jubilata, kappa and lambda. A key to the species of Bitheca is provided and their phylogeny is discussed. Pterogramma steyskali Deeming and Minilimosina masoni Marshall are moved into the genus Bitheca. A cladogram is provided and its derivation explained in detail, with a discussion of the use of character weighting in cladistics.