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Plannapus microcephalus (Haeckel, 1887)

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Dicolocapsa microcephala
Shell smooth, thin-walled, with distinct collar stricture. Cephalls small, subspherical, with small, crowded pores. Length of the two joints =1 :3, breadth 1 : 3. Thorax also nearly spherical, three times as large as the cephalis, with small, irregularly scattered, circular pores, and much broader bars.
Dimensions.-Cephalis 0.03 diameter, thorax 0.09 diameter.
Habitat.-Western Tropical Pacific, Station 225, depth 4475 fathoms.
Haeckel 1887
Dictyocephalus microcephalus
Description: Shell smooth with distinct collar stricture. Ratios of width and length between cephalis and thorax about 2:7 and 1:4, respectively. Cephalis campanulate, 36 Ám in width, 42 Ám in length, and has short V-tube. Thorax subspherical, 123 Ám in width and 163 Ám in length, and has thick and small peristome. Cephalic wall pustulated with a few scattered small pores and many small dimples. Thoracic wall slightly pustulated and has regularly ar- ranged small pores. Basal aperture elliptical.
Remarks: Haeckel (1887) overlooked the presence of basal aperture, V spine, V-tube and V-arch in this species. This species does not belong to Dicolocapsa and has characters of Dictyocephalus.
Occurrence: Loc. KN, tuffaceous mudstone of the Kinugasa Formation (middle Miocene). Reg. no. IGUT HN6035-01.
Nishimura 1990
Plannapus microcephalus
Description: Shell two-segmented without transverse pore alignment, hyaline. Cephalis distinctly lobed with paired (lateral) lobes externally visible as bulges sitting anteriorly under eucephalic lobe and posterior to small antecephalic lobe. Surface of cephalis rough, dimpled, sparsely perforate with tiny circular to subcircular, irregularly arranged pores. One odd specimen seen with smooth surfaced cephalis (pl. 6, fig. 1). Stout, conical apical horn usually present. Eucephalic lobe extends upwards into apical horn via anterior circular pore (pl. 1, figs. 10, 13, 14) and apical horn often open proximally (pl. 6, fig. 3). Well-developed vertical tube extends upward from base of cephalis at an angle varying from almost horizontal to greater than 45░. Vertical tube circular in cross-section, expressed externally as cone-shaped protrusion on posterior of shell at collar stricture (pl. 6, fig. 2). Collar stricture externally well defined by change in both contour and surface texture of shell. Thorax very inflated in lateral view (some specimens with thorax nearly circular) with smooth surface; marked lateral compression in sagittal plane. Small, circular to subcircular pores tend towards diagonal alignment although some specimens show very broad longitudinal alignment. Aperture constricted, ovate, lacking peristome. One specimen observed to have three short wings or appendages spaced equally around upper thorax, extending outwards and downwards, but this feature very unusual (pl. 1, fig. 14); wings not seen to be associated with any internal elements but align with D, LI and Lr. Internal skeleton consists of bars M, D, A, V, LI, Lr, II, lr, Vbl, Vbr, spine Vs and arches A-Vbl, A-Vbr, D-ll, D-lr, II-Ll, lr-Lr, Ll-Vbl, Lr-Vbr and Vbl-Vbr (text-fig. 3; pl. 6, fig. 4, 5). A extends freely upwards a short distance before becoming incorporated in anterior cephalic wall, finally projects from cephalis as apical horn (pl. 6, fig. 5). Small antecephalic lobe anterior to A. D extends downward at an angle to thorax. LI and Lr extend to arches then extend downward to thorax. V extends upward at angle from M to where Vbl and Vbr branch from it. Vbl and Vbr short and stout, extend laterally to join cephalic wall either side of base of vertical tube. Vs extends freely into vertical tube from junction of Vbl, Vbr and V. Bars 11 and lr extend to cephalic wall, not projecting outside. Arches A-Vbl and A-Vbr define lateral lobes (pi. 6, fig. 5), beginning about halfway up A, extending downward to join Vbl and Vbr. Arch Vbl-Vbr defines lower base of vertical tube. Remaining arches form ring at base of cephalis, though D-ll and D-lr often hidden within cephalic wall.
Dimensions (based on 20 specimens): length of apical horn: 10-33Ám; length of cephalis: 23-31Ám; width of cephalis: 26- 35Ám; length of thorax: 62-75Ám; width of thorax: 68-83Ám; width of thorax in sagittal plane: 47-54Ám (from 4 specimens).
Discussion: Nishimura (1990, p. 163, fig. 35-5) named this species Dictyocephalus microcephalus, saying that Haeckel (1887) overlooked the presence of a basal aperture and vertical tube so it did not belong in Dicolocapsa Haeckel (1881, p. 433). It is agreed that it does not belong in Dicolocapsa because it has a vertical tube and an apical horn (Nishimura's illustrated specimen does not have an apical horn) but neither does it belong in Dictyocephalus (see Discussion for Plannapus, above).
O'Connor 1997


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