Benson 1966 - Stylodictya tenuispina

The nature of the latticed concentric rings (or spiral whorls in a few specimens) which are present beyond the central region of concentric, discoidal shells in this species-group is highly variable. Three general types were observed, but the relationships between them are unclear, i.e., a few specimens show gradations between these types. The first type (Pl. 10, figs. 3,6) has closely and equally spaced rings similar to Stylodictya validispina Jørgensen. The second type (P1. 10, fig. 7) has fewer rings which are more widely spaced, and the outermost rings are suboctagonal in shape in a few specimens; in several specimens the outermost rings are incomplete, consisting of concave, outwardly curved, discontinuous portions, each attached to a single radial beam. The third type consists of very irregular rings (P1. 10, figs. 2, 4).All these forms, however, have a similar central region, i.e., concentric discoidal shells, not rings, and fully developed forms, which are generally circular to elliptical in outline, have a similar fine-pored, marginal chamber with a relatively wide pylome surrounded by numerous tooth-like spines. The latter character, however, may represent a secondary characteristic and have no specific value. All specimens have numerous radial beams, a few (as many as 10) of which are continuous to the central shell. No beams extend as free spines beyond the outer marginal chamber. The pores of the latticed plates are similar to those of Sty1odictya validispinaand Ommatodiscus pantanellii. Because the relationships between these three types are unclear, the writer considered it best to combine them into one species-group. Further study is needed in order to determine whether the three types are individual variants or represent separate species. The margin of fully developed forms has a pylome and completely covers the outer beams or conical spines arising from the outermost ring; therefore, this species-group technically belongs to the genus Ommatodiscus Stöhr (1880, p. 115).In specimens with the margin not fully developed, however, the spines or beams are free; thus they technically correspond to Stylochlamydium Haeckel (1882, p. 460) because there are usually traces of a thin, small-pored margin which represents a portion of the marginal chamber. Forms with the outermost beams or spines not developed technically belong either to Trematodiscus Haeckel (l861b, p. 841) if there are no traces of the margin or to Perichlamydium Ehrenberg (1847, p. 54) if the margin is present. On the basis of the morphology of fully developed specimens this species-group was placed within Ommatodiscus. Due to the unclear relationships between the Gulf specimens of this group and the possible inclusion of this group in four separate genera, no species name was chosen from the literature, nor is a new name proposed. Further study of type material of numerous species illustrated in the literature may reveal that they are the same as the Gulf species. The distinction between concentric discoidal shells and concentric rings in the central region of the test was not made by earlier workers.

Measurements: based on 30 specimens from stations 46, 71, and 81: diameter of complete specimens (with all or most of the marginal chamber complete) 172-317 µm; diameter of outermost concentric discoidal shell of the central region (3-7 shells) 41-113 µm; diameter of outermost ring (complete or incomplete) 123-275 µm.

Distribution: Incompletely developed specimens of this species-group cannot be distinguished from incomplete forms of many of the species of the Ommatodiscilae; therefore, the frequency of this group or of other spongodiscid speciesmay be higher than indicated. In the Gulf this species-group is absent only at stations 203 and 214.It is rare only at stations 133, 151, 191, and 192, and common at all other stations, at most of them being one of the five most abundant species. Its highest frequencies were observed at stations 95 (7.4%) and 99 (7.0%), located in or near regions of upwelling. Across the Gulf from stations 91 through 99 it has a greater frequency near the eastern and western sides but decreases in the axial part of the Gulf (station 93, 3.4%); therefore, in local areas upwelling may control its distribution. It occurs with greater average frequency in the southern half of the Gulf than in the northern half, suggesting its greater affinity for oceanic water masses; however, at a few stations in the northern Gulf, namely 184 (5.4%), 194 (4.5%), and 136 (4.0%), its frequency is similar to that in the southern Gulf, suggesting its tolerance of higher salinity and temperature. Thus, it has a cosmopolitan distribution in the Gulf but is also controlled, in part, by upwelling. Without further study of this group, including study of topotypic material, nothing definite can be stated about its world-wide distribution. It may be confined to tropical seas. \ From: Benson, 1966, p. 210-213; pl. 10, figs. 3, 5(?), (not figs. 2,4, 6-7)Ommatodiscus sp. [part.]

Remarks: I separated this species-group from Sty1odictya validispinaand Ommatodiscus pantanellii on the basis of the presence of concentric, latticed, discoidal shells in the central region of the test (text-fig. 13) rather than concentric rings present between and perpendicular to two, parallel, latticed sieve-plates; therefore, in side view the central region of the test is biconvex (P1. 10, fig. 5).Whether this character is a more fundamental basis of separation than the number and regularity of the concentric rings or discs could not be determined.

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