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Current identification/main database link: Globigerina bulloides d’Orbigny, 1826
Shape of test high trochospiral; equatorial periphery fairly lobate; axial periphery rounded. Fifteen globular chambers, increasing regularly in size, loosely arranged in slightly over three whorls. Umbilicus fairly shallow and large, near quadrangular in outline. Aperture a wide and low arch without lip; apertures of earlier chambers are visible in the umbilicus. The wall is transparent and delicate, densely and regularly covered by large pores, and with not very frequent, fine spines, circular in cross section. Around the base of the spines are cuffs of small plate shaped crystals.
Size: Largest diameter 0.403 mm; height of spira 0.404 mm.
Extra details from original publication
Paratypes: Sixty-three specimens (NMB C-26960-64, 26981-83, 26986-87, 27082-83, 27223/1-50); Leg 15, Site 147, Core 2, Section 1, top.
Dimensions (paratypes): Diameter 0.277-0.473 mm; height of spire 0.239-0.567 mm; average diameter 0.363 mm; average height of spira 0.366 mm. Number of chambers (paratypes): 11-17; average 13.67.
The height of spire is variable but always distinctly greater than in Globigerina bulloides s.l. The chambers are arranged in a loose high spire of slightly over three whorls. The chambers are globular or somewhat elongate in the last whorl and the final chamber may be tilted over the umbilicus (Plate 2, Figure 6). The wall structure is uniform. The wall is fairly thin, possesses regularly arranged large pores with a diameter larger than in Globigerina bulloides. The cuffs of small crystalls surrounding the spines at their base are clearly separated from each other. Only rarely are they enlarged giving the surface a rough appearance (Plate 2, Figures 5-6).
Differential diagnosis: The subspecies differs from Globigerina megastoma megastoma Earland, 1934 (see lectotype description of Banner and Blow, 1960, p. 14) in the wider aperture without lip and the larger number of chambers, 13 to 14 against 11. The relationship is shown in the similar high trochospiral arrangement of chambers and the similar shape of the ultimate chamber. It would appear that differing ecological conditions account for the morphologic differences between Globigerina megastoma cariacoensis and the antarctic Globigerina megastoma megastoma. The elongated shape of chambers in some specimens suggests a relation to Globigerina bermudezi, in which chambers, however, are arranged in a much lower spire and with an aperture having the tendency to be more extraumbilical in position.
Globigerina megastoma cariacoensis is readily distinguished from Globigerina bulloides s.l. The chambers are arranged in a much higher trochospire, are more loosely coiled, and increase less rapidly in size. Through this, the test becomes more circular in equatorial view and the umbilicus is distinctly wider. The aperture is much larger, but not so highly arched, and the wall is more delicate with distinctly larger pores.
Occurrence: Throughout Site 147, particularly frequent in Cores 1 through 8 (Zone X to Z of Ericson and Wollin). Late Pleistocene to Holocene.
Globigerina megastoma cariacoensis compiled by the pforams@mikrotax project team viewed: 7-3-2021
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