The Lower Cretaceous rudist genus Retha Cox and Pachytraga Paquier are recognized as the oldest members of the Family Caprinidae d´Orbigny. The oldest rudists from Mexico were described by Felix in 1891 from Barremian rocks near Zapotilán Salinas, State of Puebla: Monopleura tulae, M. Otlomitli and M. Voltani. Later also Mulleried, in 1933, nominated nine monopleurid species: Monopleura (Himeraelites) Klingharty, M. (H) Ochoterenai, M. (H) tulae, M. texcalensis, M. (H) Felixi, M. (H) mexicana, M. (H) Medellin, M. (H) zapotitlensis y M. (H) Paronai. The same specimens of these two authors were studied by Skelton and Masse in 1998, and place all of them in synomym with Retha tulae (Felix, 1891). During recent paleontological investigations in the Huetamo-San Lucas region, State of Michoacán, abundant rudists of the same genus were collected from rocks of the San Lucas (Barremian) and Comburindio Formations (Upper Barremian-Lower Aptian). After the lineaments for the classification of this species given by Skelton and Masse, the material presents the following characteristics: shell small to medium size, with blunt antero-ventral and weaker postero ventral longitudinal carinae on both valves. The right valve is elongated and straight, though somewhat twisted. The left valve is prosogyraly extended. The total height of the specimens is 60 -70 mm long, Retha tulae shows incipient development of the diagnostic synapomorphy of the subfamily Coalcomaninae Coogan.