The abundant ledges of limestone characteristic of the type Cincinnatian in the Ohio Valley can be grouped into several classes on the basis of a few criteria that are readily recognized in the hand specimen. Such a classification can make rock descriptions much simpler for those who measure sections there; furthermore, closer correspondence is now possible between the rock descriptions used or published by different workers. The petrographic properties of the 7 classes are fully described and illustrated, although the limestone types are readily distinguished as follows: 1) Fabric of whole and broken fossils randomly arranged; insoluble fraction small and localized. 2) Fabric of broken and whole fossils oriented parallel to bedding; insoluble fraction moderate in amount and localized or disseminated. 3) Fabric of whole and broken fossils, either oriented or randomly arranged; insoluble fraction large. 4) Fabric of finely broken fossil debris; insoluble fraction small and disseminated. 5) Fabric of finely broken fossil debris; insoluble fraction moderate to large; rock laminated or cross-laminated. 6) Fabric of very finely broken fossil debris; insoluble fraction large. 7) Like number 6, but occurs as nodules.