. Bulletin of the British Museum (Natural History). . Fig. 430 Pachydon iquitensis (de Greve). Pebasian; Iquitos, Peru. Copies of original illustrations of holotype, the only known specimen, a right valve, of Anisothyris iquitensis de Greve (1938: pi. 5, figs 38-41). a, internal view, x 6 (fig. 38); b, internal view, x (fig. 39); c, external view, x 2 (fig. 40); d, dorsal view, x 6 (fig. 41). Note: de Greve gave no dimensions in his text. ally :1; see de Greve, 1938: pi. 7, figs 10, 11, a re- illustration of one of Boettger's Pebas shells). P. erectus elongatus (Boettger) fro
. Bulletin of the British Museum (Natural History). . Fig. 430 Pachydon iquitensis (de Greve). Pebasian; Iquitos, Peru. Copies of original illustrations of holotype, the only known specimen, a right valve, of Anisothyris iquitensis de Greve (1938: pi. 5, figs 38-41). a, internal view, x 6 (fig. 38); b, internal view, x (fig. 39); c, external view, x 2 (fig. 40); d, dorsal view, x 6 (fig. 41). Note: de Greve gave no dimensions in his text. ally :1; see de Greve, 1938: pi. 7, figs 10, 11, a re- illustration of one of Boettger's Pebas shells). P. erectus elongatus (Boettger) from Pebas was also re- figured by de Greve (1938) and may be readily recognized by its prominent, erect, umbones. The specimen figured by Willard (1966: pi. 56, fig. 3) from 'EI Salad', north of Iquitos, as Corbula abundans Pilsbry & Olsson is misidentified. Anderson's trivial name cebada is of uncertain meaning, though the Spanish word for 'barley' could be intended. It is taken as a noun is apposition and not an adjective, and therefore not inflected. Pachydon ledaeformis (Dall, 1872) Figs 426-429 *. 1872 Corbula (Anisothyris) ledaeformis Dall: 92; pi. 16, figs 14, 15. Type material. USNM, not studied. Upper Amazon Basin, unlocalized, presumed Pebasian (Orton colln, see Remarks). Material studied. BMPD LL28068-71, Late Caenozoic, Pebasian; Pichana, Peru (Hauxwell colln, extracted from matrix, 1982). Diagnosis. Small crassatelliform Pachydon; elongate with length about twice height; umbones situated at about one third of length from anterior of shell, posterior rostrate. Description. The material is fragile and no completely undamaged shell outlines are available in the material at hand. The umbones are not large, but are sufficiently pro- duced to make both the anterodorsal and posterodorsal margins appear slightly concave when the shell is viewed externally. From inside, these margins, which form the two halves of the hinge, appear virtually straight. The shell outline has
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