Heterotrypa ulrichi

• Zoarium Morphology: Ramose to subfrondescent; diameter: 5-10 mm
• Zoecia: Rounded; 7-8 in 2mm; acanthopores numerous, usually bimodal in size
• Mesozooids: Abundant
• Monticules: Surface smooth to sharply monticulated
• Maculae: Larger zooecia and more mesopores

• Zooarium ramose to subfrondescent, 5-10mm in diameter. Surface smooth to sharply monticulated, maculae of larger zooecia and mesopores. Zooecia rounded, 7-8 in 2mm. Mesopores abundant. Acanthopores fairly numerous, of 2 sizes

Parks & Dyer (1922):

• Ulrich removed the species to his new genus Dekayella on account of the presence of two kinds of acanthopores, large and small. The former originate in the immature region while the latter appear only in the mature or superficial region.
• This species has long been considered as one of the most typical fossils from the Don river; it has been identified by more than one author and has been regarded as the chief evidence of the Eden age of these beds. While we are not prepared, in the face of many statements to the contrary, to deny the occurrence of this form, we fail to find among many dozens of thin sections any definite evidence of its occurrence. Nearly every specimen which has been ascribed in our collections to this species turns out on the microscopic examination to be Hallopora subplana. As the surface is in every instance obscured by hard shale in the apertures, the identification has been made merely from the shape of the zoarium which is indistinguishable from that of Hallopora subplana.
• While we are unable at present to include this species with certainty it is thought advisable to refer to it in view of what has been said above. Nicholson’s original figures are reproduced herewith; it will be observed that the tangential section does not show the two kinds of acanthopores.

Nicholson (1881):

• Spec. Char. – Corallum ramose, of cylindrical or subcylindrical branches, which divide dichotomously at irregular intervals, and vary in diameter from less than two lines to about four lines. The surface is smooth and destitute of monitucles but in well-preserved specimens minutely spinose; the calices sub-polygonal or rounded, mostly from 1-100th to 1-90th inch in diameter. Interspersed with the openings of the ordinary corallites are the minute irregularly rounded apertures of a largely developed series of small interstitial corallites.
• As regards internal structure, the corallites are at first thin-walled, but become thickened in the outer part of their course, their walls becoming at the same time seemingly fused together. The small angular interstitial corallites occupy all the intervals left between the oval or rounded large tubes, and there is a largely developed series of thick-walled hollow tubuli (“spiniform corallites”), placed at the angles of junction of the normal corallites. Tabulae are wanting, or are very sparingly developed in the axial region of the branches, but are abundantly present in the outer portion of the tubes, and are much more closely set in the small interstitial corallites than in the large ones. In all cases, the tabulae are complete and approximately horizontal.
• In general form and aspect, M. ulrichii most nearly resembles the larger examples of M. gracilis, James, having cylindrical stems, with a smooth surface, and comparatively minute corallites of two sizes. It is, however, distinguished from M. gracilis by the fact that the corallites do not open obliquely upon the surface, by the thinner walls of the calices, and by the grater abundance of the interstitial corallites.
• As regards its internal structure, tangential sections of M. ulrichii (fig. 22, C, D) show that the normal corallites of the colony are divided into two distinct groups, which are uniformly intermingles with one another, but differ in size and in other characters. The large corallites are mostly oval or sub-circular, mostly from 1-90th to 1-100th inch in diameter, and only partially and to a very limited extent in actual contact. Occupying all the intervals between the large tubes are numerous smaller interstitial corallites, which vary much in shape and size, but are always more or less angular. These are never so far developed as to isolate the large tubes, and there is never more than one row of them between any given pair of the latter. In addition to the preceding, there are numerous circular, thick-walled, and darkly-outlined hollow tubuli (“spiniform corallites”), which are intercalated at most of the angles of junction of the normal corallites. With the terminations of these upon the surface I am not acquainted, except in a few instances, in which they appear as a series of short spines.
• The axial region of the corallum, as seen in transverse sections, is very largely developed as compared with the circumferential portion, and the corallites are here thin-walled and polygonal in shape, while they are also almost, or sometimes quite, without tabulae. As the tubes turn outwards towards the surface, as shown in long sections (fig. 22, E), their walls thicken and become fused together; while complete horizontal tabulae are now abundantly developed. There is also seen to be a decided structural distinction between the larger corallites and the interstitial tubes, the latter being much more closely tabulate than the former. Lastly, the thick-walled “spiniform corallites” may be occasionally detected in long sections.

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