There has been a lot of confusion in both scientific and popular literature of the various genera and species within the Mellitidae over the years due to morphological plasticity and similarities in appearance (and let's face it, most people selling fossils and tests online do not care about accuracy). Fortunately, several recent phylogenetic papers aid in constructing a reliable understanding of the family.
Encope is known dating back to Miocene deposits in Mexico, South America, and the Caribbean (Coppard and Lessios 2017; Osborn and Ciampaglio 2019), and is considered a sister group to the other mellitids. Leodia, the six-keyed sand dollars, first appears with Leodia divinita in Pliocene Venezuela, while the earliest Mellita appears in the Pliocene of the Yorktown Formation (M. caroliniana; Coppard et al. 2013) and the Tamiami Formation in Florida (M. aclinensis; Mooi and Peterson 2000). Leodia sexiesperforata is known from Pleistocene fossils in North America, and has a modern range along the western Atlantic coastline from the southern U.S. to Uruguay, and throughout the Caribbean (Alencar et al. 2017). The closing of the Isthmus of Panama had a direct impact on speciation in both Encope and Mellita. Two sand dollars (granti and longifissa formerly designated Mellita from the Pacific side of Mexico and Central America were different enough (distinctive pedicellaria, etc.) to be placed in their own genus, Lanthonia (Coppard 2016); Mellita notabilis can still be found in those same Pacific waters. (I have not seen any discussion of how this generic change may affect the Pleistocene California species Mellita kanakoffi (Durham 1961).)
Along the southeastern U.S. coast, the species of five-lunuled sand dollar is Mellita tenuis (Coppard et al. 2013). This used to be called M. quinquiesperforata, but genetic research has determined that species is limited to a small section of the Central American coastline. Technically, the oldest fossil Mellita (and occasional aberrant modern specimens (Cerame-Vivas and Gray 1964)) have six lunules, but the formation of these is different from those in Leodia (Mooi and Peterson 2000).
Several modern species of Encope can be found in the waters of the U.S. and Mexico. East Coast species include E. michelini and E. aberrans (Atlantic and Gulf of Mexico), and E. emarginata in the Caribbean and Gulf of Mexico. On the Pacific side, E. borealis, E. grandis, E. californica, E. micropora, E. perspectiva, and E. wetmorei have been noted along Baja and in the Gulf of Mexico (Coppard and Lessios 2017). Osborn and Ciampaglio (2019) noted E. aberrans, E. emarginata, E. dubarorum, E. macrophora, E. michelini, and E. tamiamiensis as fossil species found in the eastern United States.
An additional genus with a single species is recognized within the Mellitidae. Mellitella stokesii is genetically distinct (Coppard and Lessios 2017), and is found in the eastern Pacific from Mexico down to Ecuador and the Galapagos Islands.
