Cladonia floridana has been misidentified as other taxa in section Perviae including C. atlantica, C. beaumontii, and C. carassensis. Mature thalli of C. atlantica can vary greatly and branching patterns are often obscured by variability. In southeastern Massachusetts, C. floridana occurs sympatrically with C. atlantica. The two species differ chemically, but are often difficult to distinguish morphologically. Cladonia floridana, which is characteristically ascyphose with a symmetrical branching pattern, may approach C. atlantica, with scyphoid openings and asymmetrical branching. In spite of the variability in mature C. floridana, it can be distinguished from other species such as C. atlantica by comparing the meristem initials, which are the primary arrangement of meristematic tissue that ultimately give rise to branches. The apical meristem of C. floridana divides in a synchronous, symmetrical pattern, but meristematic divisions in C. atlantica are asynchronous and asymmetrical. These patterns result in unique meristem initials that provide a reliable taxonomic feature. Meristem initials, which partly determine podetial morphology, are less variable than branching patterns. Suites of characters distinguish most lichen taxa and these characters have traditionally been based upon morphological and chemical states. Sometimes the gross morphology of two taxa can overlap even though their chemistries are different. In these cases, it is important to find some morphological distinctions if we are to consider the taxa as more than chemical variants of the same species. Two species that grow sympatrically in southeastern Massachusetts, Cladonia floridana Vain. and C. atlantica Evans present a problem of this sort. Cladonia floridana is a highly variable species in which branching patterns may be obscured by growth. Mature thalli of C. floridana are difficult to distinguish from related taxa in section Perviae, including C. atlantica. The chemistry of the two species differs; C. floridana contains thamnolic acid and C. atlantica contains squamatic and baeomycesic acids. Other than this character, it may be impossible to distinguish the species on the basis of their gross morphology. However, the arrangement of meristematic tissue at the youngest apices of the podetia always distinguishes these species (Hammer 1995a, 1996). Although only C. atlantica and C. floridana are found in Massachusetts, C. beaumontii (Tuck.) Vain. and C. carassensis Vain. were previously reported to occur there. The objectives of this study are to clarify which of the species occurs in Massachusetts, and to find morphological characteristics that can be used to distinguish C. floridana from C. atlantica. MATERIALS AND METHODS Approximately 200 specimens were examined, including both recently collected material and preserved herbarium material. Field and laboratory observations were made on the fresh material. The preserved material was borrowed from FH-, H, NY, TUR-V, and us, and included the type specimens (Ahti 1993) of the four species. Numerous specimens were studied and drawn at various magnifications, primarily by means of a dissecting microscope. Podetia and other thallus components were studied, and particular attention was given to young apices of podetia. Specimens were assayed for secondary chemicals using the techniques in Culberson (1974) and summarized in Hammer (1995b).