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|Discorbina simulatilis Schwager 1883|
External Characters and Internal Structure.—The genus Discorbina may be regarded as presenting the characteristic features of the Rotaline series in their simplest, as Rotalia does in their most developed, condition. Its shell is typically a turbinoid spire, formed by a succession of vesicular segments, every one of which bears a strong resemblance to a segment of a Globigerina (ex, Fig. 104 *) ; the only essential difference between the two types consisting in this, that each chamber of Globigerina has a distinct communication with the umbilical vestibule, whilst in Discorbina each chamber communicates only with that which precedes and follows it. The aperture is a large fissure, more or less arched, reaching to the lower edge of the umbilical margin of the septal plane ; and it is nearly always more or less occluded by an " astral flap," which may be a mere projection of the exogenous substance deposited on the umbilicus, or may be part of the more developed " asterigerine " plates to be presently described. The general form of the shell is conical, with a rather sharp margin and a nearly flat base. [Carpentr et al. 1862 p.203]
Extra details from original publication
352. These characters are typically shown in the Rotalia turbo of D'Orbigny, of which fossil varieties had been described by Lamarck (lviii), under the names Discorbites vesicularis, Rotalites trochidiformis, and R. lenticularis. Of these the R. trochidiformis of the Grignon Tertiaries is the largest and most strongly developed, but it is scarcely the most characteristic; the peculiar features of this type being more conspicuously exhibited by the Rotalites vesicularis of Lamarck, which is the Rotalia Gervillii of D'Orbigny's models and the Discorbina vesicularis of Parker and Rupert Jones, and which is very common at the present time on the Australian coast. The form of this shell (figs. 2, 3) is plano-convex, or concavo- convex, its umbilical surface being flattened, or even concave, whilst its upper surface is moderately arched. Its segments are vesicular externally, so that its septal bands are furrowed; and they commonly retain their globose form on their anterior extremity, which forms the posterior boundary of the next segment, and is not difl'erentiated save by the loss of its pores from the remainder of the chamber-wall. The upper surface may show the spire from its very commencement ; but as it commonly happens that each later whorl in some degree overgrows the one which precedes it, the primordial chamber is sometimes concealed. The character of the upper surface is for the most part but very little altered by exogenous deposit ; though sometimes the apex of the spire is crowned by a glistening boss of hyaline substance. The vesicular form of the segments is sometimes retained on the lower or umbilical surface, but more commonly it gives place to a flattening ; in either case, how ever, there is a deep furrow- ing at each septal junction, leading towards the umbilical depression. This depression, with its radiating furrows, is covered in by a sort of tent of hyaline non-tubular shell-substance, which sends an astral prolongation to attach itself to the wall of each of the surrounding segments (fig. 3) ; and thus a space is enclosed, which (it can scarcely be doubted) is filled in the living state with sarcode-substance. Under some condition or other, this " asterigerine " growth is found in almost every variety of the genus Discorbina, but it may be reduced on the one hand to a mere rudiment, as in the D. globularis of our own shores; whilst, on the other, it may be exuberant enough to form a solid mass, as in the fossil D. trochidiformis, where we find the surface of that mass covered with hemispherical granules, which especially run along the margins of the septal junctions, so as to give to tiiis variety a curious analogical resemblance to Rotalia Beccarii (369).
Discorbina compiled by the pforams@mikrotax project team viewed: 13-6-2021
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