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Eucyrtidium calvertense Martin, 1904

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Shell smooth, spindel-shaped, with five joints (and possibly a sixth broken away); cephalis subspherical, with a very short horn; proportional lengths of joints = 3:5:8:8:8+?; cephalis with a few scattered pores, second joint with numerous irregular pores, third joint with seven rows, fourth joint with six rows, and fifth joint with seven rows (+?) of larger regular pores.

Martin 1904
Shell spindle-shaped, with five or six joints, all strictures marked by internal transverse septa. Cephalis spherical to subspherical, with small circular pores scattered over its surface, bearing a short conical, evrtical, approximately apical spine. Thorax conical, about twice the length of cephalis, with widely,irregularly spaced circular pores of about the same size as those of cephalis. Thickness of wall of cephalis and thorax about equal, thorax in some speciemens thickened. The third, fourth and fifthe segments are of similar length, about twice that of thorax, and have a considerably thicker wall. These thicker segments have 24-30 longitudinal furrows in which lie circular pores 2 to 3 times the size of thoracic pores. Shell usually reaches its maximum breadth at the fourth segment and then tapers to a constricted mouth. Pores of last segment irregular in size and arrangement.
Dimensions: Total length 175-220, maximum breadth 90-110, length of cephalis 10-17, of thorax 20-30, of third segment 25-55, of fourth 25-40, of fifth 28-40, of sixth 35-45. Descritpion based on 65 individuals from cores V-16-116, 750cm; V-18-69, 1055cm; and V-16-66, 880cm.
Discussion: This species was first described by Martin (1904) from the Calvert Formation of Miocene age. It was also reported by Nakaseko (1955) from the late Miocene Nishidojima mudstone. Nakaseko (1959, 1960) subsequently reported the occurence of this species in various Japanese formations of Middle and Upper Miocene age. Riedel (personal communication) thinks this species may extend back to the Oligocene. Individuals resembling Eucyrtidium calvertense but having thinner shells occur in core tops taken north of the Polar Front. These individuals are so few it is difficult to know if they are reworked. Riedel (personal communication) has noticed similar thin-shelled forms in North Pacific sediment. It is possible that this species or a close relative ranges into the Recent in some areas. It does not occur in Recent sediments south of the Front but is one of the most common members of zone ϕ. Its upper limit in cores containing the red clay diatomite sequence is approximately the boundary between these lithologies (Figures 22-27).
Hays 1965


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