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Lithostrobus hexagonalis Haeckel, 1887

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Benson, 1966, p. 508-510; pl. 35, figs. 1-2:

Lithostrobus cf. hexagonalis Haeckel

?Lithostrobus hexagonalis Haeckel, 1887, Challenger Rept., Zool., vol. 18, p. 1475, Pl. 79, fig. 20.
?Spirocyrtis scalaris Haeckel, 1887, Challenger Rept., Zool., vol. 18, p.1509, Pl. 76, fig. 14.

Relatively long, broad test with 5-8 or more joints. The 4-5 joints below the cephalis campanulate but with angular shoulders coincident with their greatest breadth; joints increase in diameter distally. Distal joint of fully-developed forms cylindrical, long, with or without a distinct constriction in its proximal portion. Joints separated by distinct angular strictures; internal septal rings lacking. Pores of all joints similar, of nearly equal size, hexagonal to subhexagonal, with or without short, thin thorns at the nodes of the intervening bars, with hexagonal arrangement in transverse rows, 3-4 rows on each of the proximal abdominal joints. Pores of the thorax similar but slightly smaller than those of successive joints, arranged in 5-7 transverse rows. Surface of test generally smooth except for thin, divergent, conical spines (1-12 Ám) originating from the angular
shoulder of each joint. Cephalis cap-shaped to truncate-conical, with a few scattered thorns or spines, with pores similar to those of the thorax; not distinctly separated from the thorax; with two, three-bladed to conical, approximately equal apical and vertical horns lying in the sagittal plane, each extending from the apical and vertical bars, respectively, which are free within the cephalic cavity. An indistinct dorsal lobe merging with the thorax is separated by a pair of lateral furrows in the wall of the cephalis that are coincident with indistinct ribs corresponding to the apical-lateral arches. The dorsal and primary lateral bars extend as indistinct ribs in the thoracic wall that are prolonged as thin conical spines (5-21 Ám) which originate from the angular shoulder of the thorax. Mouth of the distal joint not constricted; terminal margin not observed complete.

Measurements; based on 18 specimens from stations 27, 46, 60, and 81: maximum length of test 123-273 Ám, maximum breadth (distal joint) 107-185 Ám; length of cephalis (to median bar) 17-25 Ám, of thorax 22-30 Ám; breadth of cephalis 22-25 Ám, of thorax 62-71 Ám; length of apical horn 2-43 mm, of vertical horn 5-32 Ám.

Remarks. Lithostrobus hexagonalis Haeckel from "Challenger" station 272 in the tropical central Pacific has joints that are similar to those of the Gulf species, but they are separated by septal rings which are located anound the broadest portion of each joint. Thin, conical spines at the broadest portion of each joint are absent in Haeckel's species. The cephalis is more nearly spherical and has only a single horn. Spirocyrtis scalaris Haeckel from ôChallenger" stations 271-274 in the tropical central Pacific resembles the Gulf species in outline of the joints, but the transverse strictures between joints are connected in a spiral. Internal septal rings are lacking as well as thin spines around the broadest portions of each joint. The cephalis is much smaller than that of the Gulf species, but it has two horns lying in the sagittal plane. Examination of the type material of both of these species is necessary before the Gulf species can be positively identified with either or both of them.

Distribution. This species is very rare but nearly cosmopolitan in the Gulf. It is absent at stations 64, 71, 99, 130, 192, 194, and all those to the north. It has a slightly but not significantly greater frequency in the southern half of the Gulf. Whether or not its occurrence in the northern half of the Gulf is dependent only on upwelling in this region cannot be determined. Its absence or very rare occurrence at marginal and northern shelf localities indicates its preference for more nearly oceanic offshore waters.
Benson 1966


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