Benson 1966 - Amphitholus acanthometra

Interior ellipsoidal shell a single system of dimensive girdles originating from a small inner ellipsoidal shell or ring-like structure. Second transverse girdle completely within the interior of shell and merges laterally with and forms a part of the sagittal ellipsoidal shell covering it and the inner trizonal shell; this shell in frontal view appears discoidal (P1. 17, fig. 5) with the two smaller dome-shaped cupolas on either side of it. Pores of the shell subcircular to subpolygonal, about equal in size, with subregular arrangement. Surface of shell generally with short thorns. In most specimens 14 regularly disposed primary beams generally continuous as radial spines are recognizable (see remarks for description); in other specimens beams thinner, more numerous, but most of the primary beams generally recognizable. Radial spines not present in all tests but present in most, their number variable from 4-100 or more, but generally only the primary spines continue inward as beams; spines conical, of variable breadth and length (5-50 µm).In a few specimens a thin, small-pored outer mantle envelops the cortical shell; it is supported by the radial spines arising from the cortical shelland is separated only a short distance from the shell.

Measurements: based on 18 specimens from stations 27, 34, and 71 (all measurements are of frontal views only): length of P1 axis 9-12 µm, of P2 axis 27-42 µm, of P3 axis (major diameter of sagittal ellipsoidal shell) 71-98 µm, of T1 axis 6-10 µm, of T2 axis 14-25 µm, of T3 axis (minor diameter of sagittal ellipsoidal shell) 42-58 µm, of maximum transverse axis (distance between tangents to the lateral cupolas) 80-121 µm, of axes S2,S3, and S4 (one specimen) 11 µm, 36 µm, and 82µm, respectively; breadth of cupolas (measured parallel to the principal axis) 55-80 µm.

Remarks: Haeckel (1887, pp. 666-668) describes several species of Amphitholus, all of which are defined on the presence or absence of radial spines and on the number of radial spines in the case of the former. The Gulf species of this genus has a variable number of radial spines, and in some specimens they are absent. The Gulf specimens, however, are generally smaller than those described by Haeckel. Because this species is rare in the Gulf, its complete range of variation could not be determined. Radial spines which appear to be primary, i.e., continuous inward as cylindrical radial beams, are generally 14 in number in the Gulf specimens. These include six mutually perpendicular spines which are arranged in pairs, oppositely placed, and coaxial with the three dimensive axes of the test, and eight diagonal spines corresponding to those observed in a few specimens of Tetrapyle octacantha. In other specimens, radial beams are thinner, more numerous, and are less regularly disposed, in a few specimens not continuous as radial spines. Haeckel illustrated only three species of Amphitholus of which A. acanthometra is most similar to the Gulf species although it is slightly larger and has 20 regularly disposed spines, six of which may be secondary. The other species described by Haeckel may be identical to the Gulf forms, but without their illustrations it is impossible to make this decision. A few specimens of the Gulf species have a thin outer veil covering and conforming to the shape of the cortical shell. Although Amphitholonium tricolonim Haeckel is slightly larger than the Gulf specimens, it was included in the synonymy. Additional remarks (Benson, 1966, p. 257).The genus Amphitholonium Haeckel becomes a junior objective synonym of Amphitholus Haeckel because its type species, Amphitholonium tricoloniumHaeckel, is placed in synonymy with a species from the Gulf identified as Amphitholus acanthometra Haeckel. Because the development of the outer mantle is subject to intraspecific variation, it cannot be used as a character defining a genus.

Distribution: This species is rare in the Gulf but occurs as far north as station 151, being absent at all those to the north as well as at station 90.Its rare occurrence prohibits analysis of control of its distribution by upwelling. Because it has a much greater frequency in the southern half of the Gulf, it is considered an oceanic species. Amphitholus acanthometra and Amphitholonium tricolonium were reported by Haeckel from the South Pacific at Challenger stations 300 and 302, respectively, in waters of the Humboldt Current off the coast of Chile. Thorough study of the taxonomy of Haeckel's species of both genera is needed before conclusions regarding the world-wide distribution of this group can be made. \ From: Benson, 1966, p.258-260; pl. 17, figs. 4-7:Amphitholus acanthometra HaeckelAmphitholus acanthometra Haeckel, 1887, Challenger Rept., Zool., vol. 18, p. 667, Pl. 10, figs. 5-6.Amphitholonium tricolonium Haeckel, 1887, Challenger Rept., Zool., vol. 18, p. 669, Pl. 10, fig. 7.

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