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Description

Benson, 1966, p. 408-410; pl. 27, fig. 6; pl. 28, fig. 1:  Pterocanium praetextum (Ehrenberg)  Lychnocanium praetextum Ehrenberg, 1872a, Akad. Berlin Monatsb. (1872), p. 316; 1872b, Akad. Berlin Abhandl. (1872), Pl. 10, fig. 2. Pterocanium praetextum (Ehrenberg), Haeckel, 1887, Challenger Rept., Zool., vol. 18, pp. 1330-1331; Riedel, 1957, Swed. Deep-Sea Exped. Repts., vol. 6, fasc. 3, pp. 86-87, Pl. 3, figs. 1-3. Pterocanium virgineum Haeckel, 1887, Challenger Rept., Zool., vol. 18 p. 1330, Pl. 73, fig. 6.  Remarks. This species does not differ appreciably from its description by Riedel (1957, pp. 86-87). The pores of the cephalis are not generally infilled but are small. The thoracic pores are more nearly equal with regular arrangement in longitudinal rows, but several specimens have less regular pores similar to those illustrated by Riedel (op. cit., Pl. 3, figs. 1-3). The feet of the Gulf specimens show a wider range of variation than indicated by Riedel. Although they are predominantly subparallel, in a few specimens they are straight but divergent outward.  This species differs from P. prosperinae Ehrenberg from the Gulf in the presence of three swellings or angular shoulders of the thorax that are each located between the indistinct thoracic ribs which occupy slight furrows in the thoracic wall. The cephalis of P. praetextum is separated from the thorax by a more distinct stricture, each blade of the three-bladed feet is latticed for a greater distance in its proximal portion, and the outward projecting blade of each foot originates from higher up on the thorax and is convex outward proximally, merging with the outline of the thorax. Four small, nearly circular collar pores separated by thick bars present a different aspect from those of the preceding species. The abdomen is similar to that of Pterocanium prosperinae and is separated from the thorax by a distinct constriction and an internal septal ring.   Measurements; based on 15 specimens from stations 27, 34, 46, 56, and 60: length of cephalis 17-25 µm, of thorax 49-59 µm, of abdomen 0-31 µm; breadth of cephalis 18-27 µm, of thorax at level of shoulders 79-92 µm; length of apical horn 31-53 µm, of vertical spine 4-10 µm, of feet 49-166 µm; maximum distance between tips of feet 49-166 µm.   Distribution. This species is very rare in the Gulf, occurring only in the southern half of the Gulf at stations 27, 34, 46, 56, 60, 64, 81, 92, 93, and 95. It is, therefore, an oceanic species with little affinity for Gulf waters.  According to Riedel (1957, p. 87), P. praetextum occurs in the upper parts of most Pacific and Indian Ocean sediment cores from tropical regions and is apparently widespread in Recent tropical seas. Its earliest known occurrence is probably upper Pliocene because it occurs together with P. prismatium Riedel in the upper Pliocene parts of Swedish Deep-Sea Expedition cores 58 and 62. Riedel (op. cit., p. 88) states that P. prismatium may be an indicator of a time-zone near the top of the Tertiary. Riedel, Bramlette, and Parker (1963, p. 1238) state that both P. prismatium and Eucyrtidium elongatum peregrinum became extinct at approximately the same time at the end of the Pliocene, although the former may extend a little later in time than was previously thought.

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