Pyrgulina densecostata
Encyclopedia
Pyrgulina densecostata is a species
of very small sea snail
, a pyramidellid
gastropod micromollusk
. This genus is currently placed in the subfamily Chrysallidinae
of the family
Pyramidellidae
.
are almost completely obliquely immersed in the first post-nuclear whorl. The eight post-nuclear whorls are rounded, rather inflated, and moderately shouldered. They are marked by many well-developed, regular, rounded, toward the aperture
slanting axial ribs, of which twenty occur upon
the second, twenty-four upon the fifth, and thirty upon the penultimate whorl. These ribs are somewhat thickened at their posterior extremity and give the summits of the whorls a beaded appearance. The intercostal spaces are a little wider than the ribs, crossed by many incised spiral lines, which are about as wide as the raised spaces between them. These incised spiral lines are a little less strongly developed on the posterior portion of the whorls, where the ribs are thickened, but anterior to these thickenings they are very regular and regularly-spaced. There are eighteen on the fifth and twenty-six on the penultimate whorl. The periphery and the base of the body whorl
are well rounded, the latter marked by the strong continuations of the axial ribs, which
extend almost undiminished to the umbilical region. The intercostal spaces on the base are marked like those between the sutures by twenty-two incised spiral lines. The sutures are well marked. The aperture is moderately large. The posterior angle is acute. The outer lip
is thin, showing the external sculpture
within. The junction of columella and the outer lip is well rounded. The columella is decidedly curved, thin, somewhat revolute, provided with a prominent oblique fold, a little anterior to its insertion. This fold joins the columella in such a manner as to give this a decidedly sigmoid curve. The parietal wall lacks a callus.
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of very small sea snail
Snail
Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...
, a pyramidellid
Pyramidellidae
Pyramidellidae, common name the pyram family, or pyramid shells, is a voluminous taxonomic family of mostly small and minute ectoparasitic sea snails, marine heterobranch gastropod molluscs or micromolluscs....
gastropod micromollusk
Micromollusk
A micromollusk is a descriptive term for a shelled mollusk which is extremely small, even at full adult size. The word is usually, but not exclusively, applied to marine mollusks, although in addition, numerous species of land snails and freshwater mollusks also reach adult size at very small...
. This genus is currently placed in the subfamily Chrysallidinae
Chrysallidinae
Chrysallidinae is a taxonomic group of very small sea snails in the family Pyramidellidae.- Taxonomy :Chrysallidinae has been one of eleven recognized subfamilies of the gastropod family Pyramidellidae...
of the family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
Pyramidellidae
Pyramidellidae
Pyramidellidae, common name the pyram family, or pyramid shells, is a voluminous taxonomic family of mostly small and minute ectoparasitic sea snails, marine heterobranch gastropod molluscs or micromolluscs....
.
Description
The very thin shell is elongate-ovate, subdiaphanous, milk-white, and shining. It measures 4 mm. The nuclear whorlsWhorl (mollusc)
A whorl is a single, complete 360° revolution or turn in the spiral growth of a mollusc shell. A spiral configuration of the shell is found in of numerous gastropods, but it is also found in shelled cephalopods including Nautilus, Spirula and the large extinct subclass of cephalopods known as the...
are almost completely obliquely immersed in the first post-nuclear whorl. The eight post-nuclear whorls are rounded, rather inflated, and moderately shouldered. They are marked by many well-developed, regular, rounded, toward the aperture
Aperture (mollusc)
The aperture is an opening in certain kinds of mollusc shells: it is the main opening of the shell, where part of the body of the animal emerges for locomotion, feeding, etc....
slanting axial ribs, of which twenty occur upon
the second, twenty-four upon the fifth, and thirty upon the penultimate whorl. These ribs are somewhat thickened at their posterior extremity and give the summits of the whorls a beaded appearance. The intercostal spaces are a little wider than the ribs, crossed by many incised spiral lines, which are about as wide as the raised spaces between them. These incised spiral lines are a little less strongly developed on the posterior portion of the whorls, where the ribs are thickened, but anterior to these thickenings they are very regular and regularly-spaced. There are eighteen on the fifth and twenty-six on the penultimate whorl. The periphery and the base of the body whorl
Body whorl
Body whorl is part of the morphology of a coiled gastropod mollusk.- In gastropods :In gastropods, the body whorl, or last whorl, is the most recently-formed and largest whorl of a spiral or helical shell, terminating in the aperture...
are well rounded, the latter marked by the strong continuations of the axial ribs, which
extend almost undiminished to the umbilical region. The intercostal spaces on the base are marked like those between the sutures by twenty-two incised spiral lines. The sutures are well marked. The aperture is moderately large. The posterior angle is acute. The outer lip
Lip (gastropod)
In the shell of a gastropod mollusk , the margin of the aperture is called the lip or the peristome. In other words, this is a term used to describe part of gastropod shell anatomy....
is thin, showing the external sculpture
Sculpture (mollusc)
The sculpture of a mollusc shell is the three-dimensional ornamentation on the outer surface, as distinct from the basic shape of the shell itself or colouration. Sculpture may be concave as well as convex. Sometimes it has microscopic detail. Sculpture refers to the calcareous outer layer, not the...
within. The junction of columella and the outer lip is well rounded. The columella is decidedly curved, thin, somewhat revolute, provided with a prominent oblique fold, a little anterior to its insertion. This fold joins the columella in such a manner as to give this a decidedly sigmoid curve. The parietal wall lacks a callus.