- 1875 Palaeophyllum divaricans Nicholson, Geological Survey of Ohio, Paleontology 2, p. 220, pl. 22 figs. 10a-b.
- 1909 Streptelasma divaricans Foerste, Bulletin of Science Laboratories at Denison University, no. 14, p. 307, pl. 10 fig. 4, a-e.
- 1924 Streptelasma divaricans Foerste, Upper Ordovician faunas in Ontario and Quebec, p. 67, pl. 2 fig. 5a-d.
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- C5 Sequence (Lower Whitewater, Liberty, Waynesville)
Identification in Hand Sample
- Maximum lengths and diameters slightly more than 30-15 mm
- Base attachment on cardinal side
- Major septa are generally wavy and might twist counterclockwise
- Tabulae apparent beginning early in development
Streptelasma divaricans from the Whitewater Formation of Richmond County, Indiana (CMC 54900)
- S. divaricans is a relatively small species. Maximum lengths and diameters are slightly more than 30 and 15 mm, respectively. External form is typically ceratoid, and curvature is usually slight. A base of attachment is present on the cardinal side. Coralla generally occur individually, but 26 percent of 189 specimens consist of two or more in lateral contact. The greatest number seen in a single group is 13. Most of these clusters represent pseudocolonies in which the corals grew into lateral contact or were attached one to another. True coloniality resulting from lateral and peripheral asexual increase is rare. The septa are generally nondilated throughout ontogeny. In early stages, the major septa extend to or almost to the axis. In late stages, the nature of the axial region varies considerably among corals. There is complete gradation from moderately complex axial structures of septal lobes and lamellae to simple structures having septal lobes only, to major septa extending to the axis without forming a structure, to speta withdrawn from the axis leaving an open axial region. Specimens with major septa extending to the axis are most frequent. A calicular boss, if developed, is slightly convex. The major septa are generally wavy and sometimes twist in a counterclockwise direction. They often meet in several groups at the axis. The cardinal septum cannot be distinguished from the other major septa, and a fossula is not developed. Minor septa are confined to or extended a short distance beyond the stereozone, which is generally narrow. Tabulae appear in very early stages and are mostly complete and moderately convex upward.
- In the Whitewater member of the Richmond of Ohio, Indiana, and Kentucky, is a small species of Streptelasma, mostly 12 to 20 mm long, which tends to grow in clusters, and with its base spreading over a relatively large attachment area. The small specimens of Streptelasma in the Liberty member of the Richmond may belong to another species. Usually, their area of attachment is much more oblique, and the corals do not rise far above their support. Small Streptelasma at the Coral horizon of Snake island resembles S. divaricans, but cannot be referred definitely to this species.
- When many S. divaricans are attached to the same support they tend to be parallel, as though rising vertically from their attachment areas.
- In the Liberty, near Clarksville, Ohio, is an undescribed form of Streptelasma rarely exceeding 10 mm. in length, attached along its cardinal side very obliquely for half or almost all its length, attached along its cardinal side very obliquely for half or almost all its length, similar to S. parasiticum described by Ulrich from the Platteville and Decorah members of the Black RIver formation in Minnesota.
- Streptelasma divaricans appears to be a small, sessile species, attached to shells or other objects. Usually, two or three specimens are attached to the same shell, at about the same point, but sometimes more than a dozen may be found in the same cluster. The individual corals are inverted conical in shape. Where growing in clusters, the sides usually are more or less adnate, and may be deformed by pressure. The area of attachment usually is more or less oblique to the base, preserving the conical form of the coral on its free side. Occasionally a radicular expansion of the edges of the area of attachment is noticed. Specimens may be found in which the corallites are free at the top, but the presence of lateral gemmation has not been demonstrated in any specimens at hand.