504 GEOLOGY AND PALEONTOLOGY OF CANAL ZONE


Gatun shells, the ribs, except the posterior ones, are divided on the lower third to lower half of the valves, depending on the size. The nominate subspecies itself occurs in the Ag-ueguexquite formation of the Tehuantepec area (Perrilliat Montoya, 1960, p. 14, pl. 1, figs. 1, 2; length 95 mmn). If the Agueguexquite formation is of middle Miocene age, the nominate subspecies is a Caribbean migrant in southeastern United States, where it is unknown earlier than late Miocene. A. floridana, that is, A. lie'nosa secticostata, is the type of the subgenus Sectiarca (Olsson, 1961, p. 97).
 Occurrence: Upper part of Gatun formation (middle Miocene), eastern area, locality 175. Middle Miocene deposits, Tehuantepec area, M6xico.
 Group of auriculate species
 Anadara (Rasia) cf. A. exuarginata (Sowerby) Plate 77, figures, 1, 2
 Small, elongate (height 64 percent of length), very inequilateral (umbo almost at anterior fourth of length), strongly inflated, auriculate. Ventral margin sloping downward toward posterior end, slightly inisinuated. Umbo sulfate. Sculptured with 30 ribs. Median ribs narrower than space between them. Anterior ribs and lower part of anterior median ribs weakly divided. Concentric threads between ribs greatly subdued. Anterior ribs weakly noded, median ribs almost smooth, posterior ribs, as usual, smooth. Cardinal area very narrow; posterior limb of a ligament groove only.
 Length 20.9 mm, height 13.4 mm, diameter 6.3 mmn (figured specimen).
 A small, presumably immature (and therefore not named), left valve, from the upper part of the Gatun formation in the western area, is the only auriculate anadarine. It has no fossil or living allies in the Caribbean region, but is allied to the living eastern Pacific Anadara envzrginata (Sowerby) (Maury, 1922, p. 23, pl. 2, figs. 5, 10; Olsson, 1961, p. 91, pl. 8, figs. 6, 6a). Although the number of ribs is the same or about the same, the posterior ribs of the living species are wider than those of the Gatun fossil and the anterior and median ribs are noded. A. emctrginata reaches a length of almost 50 mm.
 The slight insinuation of the ventral margin of the Gatun fossil indicates a gape. An exceptional paired right and left valve of A. emarginata (USNM 716408; Santa Elena, Ecuador; length 32.5 mmn) has a distinct gape and extraordinarily narrow median ribs in line with the gape. Its six ligament grooves are bent, rather than angulated, a little behind the immbo.
 The byssus of immature and mature A. auriculatathat is, A. notabilis, a western Atlantic auriculate species-was described by Perry (1940, p. 29, pl. 1, figs.


6a, 6b), and her figure 6a shows the mature byssus. She was mistaken, however, in mentioning a byssal notch.
 Occurrence: Upper part of Gatun formation (middle Miocene), western area, locality 184.
 Group of short species
 Many species show gradations between the arbitrarily divided groups of elongate and short species. The early Tertiary species of both groups in the faunas under consideration are small.
 Anadara (Rasia) species
 Small, short (height about 78 percent of length), very inequilateral (umbo a little behind anterior fourth of length). Sculptured with about 24 undivided ribs. about twice as wide as space between them, except at anterior and posterior ends. Cardinal area inaccessible.
 Length (practically complete 14.1 mm, height (incomplete) 9 mmn (estimated restored height 11 mm), diameter 6 mm.
 This unnamed species is represented in the marine member of the Bohio (?) formation by two poorly preserved right valves, the larger incomplete and the other very small (length .5 mm). The ribs of the larger valve are wide for the size of the shell.
 Occurrence: Marine member of Bohio (?) formation (late Eocene), locality 40d.
 Anadara (Rasia) lita Woodring, n. sp.
 Plate 70, figures 1, 2, 6, 13
 Small, short (height 77 to 79 percent of length), inequilateral (umbo near anterior third of length), strongly inflated. Umbo slightly sulcate. Sculptured with 22 to 24 ribs. At early stage ribs about as wide as space between -them, later a little narrower than space. Ribs of right valve weakly or faintly noded; a few near anterior end weakly divided near ventral margin on type. Ribs of left valve strongly noded, except those on posterior area. Cardinal area wide for size of shell; a pair of ligament grooves and an additional posterior limb on type.
 Length (not quite. complete) 18 min, height 14.2 mm, diameter 7.5 mm (type).
 Type: USNM 646695; paratype 646854.
 :Type locality: 42d (USGS 18837, Barro Colorado Island, northern part of island, stream'heading west of Miller Trail near Miller. 17, about 100 meters above mouth, Canal Zone), Bohio formation.
 The upper parft of the Bohio formation on lBarro Colorado Island yielded two right and four left valves of A'nadara lita. A more elongate right valve, (length 10.1 mm, :height 6.6 mmi) fromt the sami-e locality is doubtfully referred to it.
 A. maisissippiensis (Conrad) (Sheldon, 1916, p. 32, pl. 7, figs. 12-16, as Area leszteuri (Daill) ; lByram marl, Mississippi) is more elongate; the ribs of the left valve


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