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Phormostichoartus fistula Nigrini, 1977

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Description: Shell thick-walled, smooth, subcylindrical, consisting of four segments. Cephalis approximately spherical with well-developed, poreless vertical tube lying along thorax for about half thoracic length; few subcircular pores; no apical horn. Collar stricture indistinct.
Thorax short, truncate conical with two or three transverse rows of relatively large subcircular pores. Lumbar and post-lumbar strictures distinct but not pronounced. Sometimes post-lumbar stricture marked by smooth poreless band. Abdomen annular, elongate with five to eight closely spaced transverse rows of large subcircular pores. Fourth segment approximately same wi dth as or narrower than abdomen ; pores similar in size and shape to those on abdomen, usually in three or four transverse rows. Segment narrowing to poreless peristome, sometimes with small, poorly developed terminal teeth. In some specimens fourth segment is elongate wi th more pore rows, often becoming irregular in arrangement and shape.
Dimensions (based on 15 specimens): Total length 110-190µ; length of cephalis and thorax 35-40µ; length of abdomen 35-53µ; length of fourth segment 35-70µ; maximum breadth 65-83µ.
Range and abundance: Rare or very rare from Thyrsocyrtis bromia Zone to Spongaster pentas Zone. Eocene and Oligocene specimens usually incomplete, with fourth segment either broken or not originally developed. Characteristic vertical tube always well developed.
Phylogeny: Uncertain; may be ancestor of Phormostichoartus corbula.
Type specimens: Holotype. 77B-10-4, B-R40/4; paratype. 77B-13-2. A-P14/2.
Nigrini 1977











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