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Amphiplecta tripleura Funakawa, 1995

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Amphiplecta tripleura
Description : Shell is composed of two segments. Internal skeletal structure is composed of A, MB, D, two L, V, two AL, two D-AL, two VL and A'. A' forms A-(D-AL). L-R is absent. V is thinner than the other internal spicules. Two L-Rd are short. A forms a long apical rib and a short apical horn. V forms a ventral horn. D and two L are well developed and form three dorsal and lateral ribs. First segment is cylindrical with long and distinct apical rib extending to apical horn. Apical horn is not bladed, short and narrow. Ventral horn is not bladed and extends obliquely upward. Its length is one to two times that of apical horn. A' is well-developed and diverges from A at the junction of A and AL. A' forms A-(D-AL); it shifts a little to right or left side from the piane of bilateral symmetry and so it connects with D-AL at near the junction with D (Plate 1, figs. 1c, 3b). Axial projection is distinct. Pores in the first segment are circular to subcircular and are not aligned. Pore bars are very thin. First segment is distinguished from second segment by two VL, a part of both two AL and two D-AL. Two D-AL are distinct. On the underside of both L and D, there are two or three small and irregularly arranged projections (Plate 1, figs. 1b, 1c). On the lateral side of A, there are one to two pairs of trigonal projections (Plate 1, fig.1c). There are a pair of lateral projections (l ?) on D that are trigonal to thorny (Plate 1, figs. 1b, 2). Second segment is expanded horizontally. In most specimens, lateral and dorsal ribs are indistinct. Three feet are not developed. Pores in the second segment are circular to subcircular and not aligned and pore bars are very thin. Distal end of the second segment is fully open.
Dimensions (in Ám) : Measurements are based on 19 specimens. Outside diameter of first segment is 45-92 (average is 59), inside width between each D-AL is 56-74 (64) and inside depth between A-(D-AL) and VL is 44-58 (51), maximum diameter of second segment is 77-136 (111). Total height of shell is 83-168 (106).
Remarks : This species has different internal skeletal structure from A. acrostoma HAECKEL (FUNAKAWA, 1994, p. 462), this species has D-AL at the dorsal to lateral side of shell but A. acrostoma has no D-AL.
Etymology : The specific name means "three ribs" in Latin.
Occurrences : MW-125, -120 and -118. Few in abundance.
Stratigraphic notes : Lower Miocene.
Funakawa 1995


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