A new genus of mellitid sand dollar (Echinoidea: Mellitidae) from the eastern Pacific coast of the Americas

Lanthonia gen. nov. Coppard 2016 is a genus of clypeasteroid sand dollar whose members inhabit shallow, sandy waters from Mexico (including the
Gulf of California) to Colombia in the tropical and subtropical eastern
Pacific. Lanthonia includes Lanthonia longifissa (Michelin, 1858) and
Lanthonia grantii (Mortensen, 1948), with L. longifissa hereby designated
as the type species. Both L. longifissa and L. grantii were previously
placed in the genus Mellita (L. Agassiz, 1841). However, levels of genetic
divergence between a lineage containing L. longifissa and L. grantii and a
lineage containing all other species of Mellita, including the type
species M. quinquiesperforata (Leske, 1778), indicate genus level
differentiation. The systematic interpretation of this group also supports
the designation of this new genus as it allows the tree topology to be
recovered from the nomenclature and clarifies the historical biogeography
of these clades. This has resulted in members of both lineages today being
sympatric in the eastern Pacific. Members of Lanthonia are morphologically
differentiated from the type species of Mellita and all Pacific Mellita in
having very narrow ambulacral regions between the food grooves and the
ambulacral lunules on the oral surface, these being very broad in both M.
quinquiesperforata and M. notabilis. The dentation of the bidentate
pedicellariae also differentiate these genera, with small peripheral teeth
present along the edge of the blade in species of Lanthonia and one to
three enlarged intersecting teeth present distally in all species of
Mellita.