. Bulletin of the British Museum (Natural History). 359a. 359b 361a 361b 1' Figs 358-361 Pachydon obliquus Gabb. Pebasian; Pichana, Peru; Hauxwell Colin. 358, L27703; right valve internal, x 4. 359, L27705; a, b, bivalved specimen viewed from left and front, x 3. 360, L27704; left valve internal, x 4. 361, L27706; a, b, juvenile specimen viewed from left side and front, x 15. Iquitos; Rumi Tuni, on Rio Napo, 225 km north of Iquitos; 100 km south of Contamana on Rio Ucayali (all Willard 1966). Late Caenozoic, Pebasian, Upper Amazon Basin only. Diagnosis. Moderate-sized Pachydon with v
. Bulletin of the British Museum (Natural History). 359a. 359b 361a 361b 1' Figs 358-361 Pachydon obliquus Gabb. Pebasian; Pichana, Peru; Hauxwell Colin. 358, L27703; right valve internal, x 4. 359, L27705; a, b, bivalved specimen viewed from left and front, x 3. 360, L27704; left valve internal, x 4. 361, L27706; a, b, juvenile specimen viewed from left side and front, x 15. Iquitos; Rumi Tuni, on Rio Napo, 225 km north of Iquitos; 100 km south of Contamana on Rio Ucayali (all Willard 1966). Late Caenozoic, Pebasian, Upper Amazon Basin only. Diagnosis. Moderate-sized Pachydon with very incurved, prosogyrous umbones; generating curve oval; both valves Glossus-shaped, but with weak posterior ridges; highly in- equivalve, with left valve much the smaller and right valve margin overlapping left; cardinal tooth of right valve very large, hooked, elongate, with correspondingly shaped socket in left valve; cardinal tooth not visible from outside when valves are closed; umbonal cavities present under hinge plates in both valves. Description. The right valve is very tumid, the left valve has about half its convexity. The umbones, particularly that of the right valve, are prominent and very strongly incurved. Growth lines suggest that the right valve describes about one and a half whorls and the left only one whorl during growth to maximum convexity. Both figures are very high for bivalves in general. Lunulc-likc swellings occur under and anterior to the umbones in both valves, and the anterior surface of the cardinal tooth is not visible from the outside as it is in several other species assigned to the genus. Two posterior ridges are developed in each valve, those in the left being much the stronger. One ridge marks the posterior angulation, the other lies dorsal to it on the posterior area and is so weak that it can be seen only with difficulty. The highly polished shell surface is smooth, except for growth lines which give rise to some wrinkling. The commissure
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