Observations on the Genus Miltha (Mollusca: Bivalvia) with Notes on the Type and the Florida Neogene Species

Abstract

The genus Miltha H. & A. Adams, 1857, has been the subject of much misunderstanding and misinterpretation, largely as a result of the rarity of the type species, "Lucina" childrenae Gray, in museum collections. Described from "Brazil" by Gray, the species was confused by Carpenter (1864) and following him, Dall. (1901, 1903) with the species from Baja California subsequently named Miltha xantusi by Dall (1905). At the time of his monograph on "The Tertiary Faunas of Florida” Dall had seen no specimens of the type species and treated Miltha as a subgenus of Phacoides "Blainville," giving it an unjustifiably broad interpretation that was uncritically accepted by a number of subsequent authors. As a result fourteen species from the Tertiary of eastern North America have been referred to Miltha, only two of which, M. chipolana (Dall) and M. caloosaensis (Dall), are here accepted as correctly referred to that genus, with two others, M. heracleus (Dall) and M. smithwoodwardi (Maury) being placed in the synonymy of M. chipolana. The other ten species are here referred to Armimiltha, Eomiltha, Plastomiltha or Saxolucina. Miltha s.s. includes species that, in the adult stages, differ in degree of inflation of the opposite valves; in the species here studied either valve may be inflated with no predominance of one over the other observed in any species. The valves have a fine concentric ornamentation with a weak and obscure radial ribbing. Anterior and posterior dorsal areas are defined by incised grooves; the posterior is well developed, but the anterior may be rather obscure. The lunule is small, confined to the right valve and more or less deeply impressed into the area immediately below the umbos. The adult hinge includes two cardinal teeth in each valve, the posterior right cardinal and the anterior left one are bifid, the others simple, more or less lamellar. There is no trace of lateral teeth in the adult hinge, but both anterior and posterior laterals are present in small, immature forms. These same forms also show a well developed escutcheon, a remnant of which persists in the adult valves as a flat, plate-like protrusion of the right valve that is partly broken away by the action of the enlarging fibrous portion of the ligament. The interior of the valve bears ridges and grooves that reflect structures in the mantle area including the attachment area of the gill demibranchs, the trend of the large pallial blood vessel and, apparently the presence of a "non-ridged fold” margining a region of "mantle gills" such as have been described in other lucinid genera by Allen (1958) and others. The anterior and posterior adductor scars show clear evidence that these muscles included both "striated" and "smooth" types of muscle fibers- as do other species of Lucinidae. The two recent species of Miltha, M. childrenae and M. xantusi both occur in waters of 28 to 30 fathoms or more in depth, and both appear to prefer fine sandy types of bottom sediment. The oldest species of Miltha s.s. appears to be M. packi (Dickerson) from the Domengine Formation, middle Eocene, of California. "Phacoides" montensis Cossmann is a possible ancestral type. No certain upper Eocene or Oligocene forms have yet been recognized, but the genus is present in the lower Miocene of California, Florida, and New Zealand, persisting to the Recent in tropical waters of the eastern Pacific and the western Atlantic Oceans. Three species of Miltha are present in the Florida Neogene deposits: M. chipolana Dall, from the early Miocene Chipola Formation; M. caloosaensis Dall, from the upper Miocene Pinecrest beds, and the Pliocene Caloosahatchee Formation; and M. carmenae, new species, from the early Pleistocene "Unit A" beds from the region south and south east of lake Okeechobee, Florida.