Middle Ordovician – Silurian
Cyclonema is an extinct genus of faculatively mobile epifaunal coprophage-deposit feeders
Identification in Hand Sample:
- Turbiniform or trochiform with spiral ornamentation
- Rapidly increasing size from apex to aperture; few volutions
- Oblique aperture with reflected inner lip
- Short spire with ventricose whorls
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- Transverse and spiral ornamentation present; Ranges from the Middle Ordovician to Silurian and is represented by several medium sized species in Ordovician rocks of Ohio.
Caster, Dalve & Pope (1961):
- In the Cincinnatian, only a few genera of snails are common. Strangely, the shells of certain genera, for example Cyclonema, are usually found complete; whereas only the internal molds of other genera, Lophospira, are commonly found. This undoubtedly reflects an original difference in the physical or chemical structure of the shell.
Treatise on Invertebrate Paleontology, Part I, Mollusca 1 (1960):
- Basically Turbiniform or trochiform but variously modified by stationary coprophagous habit; always with characteristically sharp spiral and finer collabral threads
- Shell turbinate, non-umbilicated, with short spire, composed of few more or less ventricose whorls. Aperture oblique with reflected inner lip, always excavated. Surface with spiral lines and small ridges crossed obliquely by sharp lines of growth.
Ulrich & Scofield (1897):
- Shell turbinate or conical, never thick, composed of few more or less ventricose whorls; no umbilicus; surface sculpture consisting of numerous revolving lines and small ridges crossed obliquely by sharp lines of growth; aperture oblique, varying from rounded to subquadrate; inner lip more or less thickened, reflected, always excavated. Types, C. bilix Conrad and C. mediale Ulrich.
- The principal characteristic of this genus is the excavation of the inner or columellar lip. This peculiarity distinguishes the genus at once from Gyronema which includes some otherwise not very different shells. In Strophostylus there is a fold on the inner lip, forming a similar excavation, but it is generally so much twisted that its upper end is not visible in a direct view of the mouth. As a rule also the inner lip is much thicker than in Cyclonema. It seems a little strange that the relations of Cyclonema and Strophostylus have not been recognized heretofore. They are certainly very closely related and in practice it is often difficult to distinguish them. But if we are correct in deriving Cyclonema from Gyronema then the two groups must have different roots. The oldest known shells (Stones River group) that are strictly referable to Cyclonema have relatively coarse revolving ridges. In our Strophostylus textilis, on the other hand, the revolving lines are very delicate. The difference is so marked that we cannot believe that Strophostylus came from the same immediate root as Cyclonema. In our opinion they represent two independent lines.
- The composition of the shell of Cyclonema must be different from that of the majority of Lower Silurian Gastropoda. On the hills about Cincinnati, where thousands of specimens have been collected, the test is preserved when all the other shells occur as casts of the interior only. Indeed, we have never seen a natural cast of Cyclonema. Another point worthy of notice is the extreme rarity of specimens retaining the apical nucleus. Out of considerably over one thousand good shells before us only six retain the apex entirely. In nearly all of the other cases the evidence at hand seems to show that these minute whorls were lost during the life of the shell, or, at any rate, before fossilization, while in four of the six specimens preserving them they were covered and protected by an encrusting bryozoan.
- Shells turbinate; thin; spire short, consisting of few volutions which increase rapidly from the apex; aperture large, rounded anteriorly, and somewhat flattened on the columellar side ; umbilicus none; surface strongly marked by spiral threadlike striae, which are cancellated by finer striae.
- The shells of this genus include forms which have been referred to Pleuromaria littorina but which have no slit or indentation in the outer lip, or band upon the volution. The surface is marked by elevated striae parallel to the direction of the volutions, and the spaces between these are marked by finer stria crossing the others obliquely; the latter, however are often obsolete.
- The Pleurotomaria bilix of CONRAD, may be regarded as the type of this genus.